Took a Level In Dumbass

"Caboose: I know where you can find O'Malley. He lived inside my helmet for a while, maybe he left an address to send his mail. We were like roommates.

Sarge: Sounds like he took some of the furniture when he left. And the carpet. And the drapes. And I wouldn't expect to get that deposit back, if you know what I mean."

- Red vs. Blue, "Episode 39"

A specific subtrope of Flanderization (and occasionally Jerkass also). As a series progresses, characters sometimes become more and more stupid as time goes on. Usually the reason is related to the Badass Decay phenomenon, but with the character's level of intelligence rather than his ability to kick ass, following something along the following cycle: an episode in which the characters acts like a total buffoon gets good ratings, so another one is made. It too gets good ratings. So they make another. And another. Eventually the character is so dumb that you wonder how he manages to walk across a room without injuring himself, and pretty soon everyone forgets that the character was anything other than a complete moron.

Generally Played for Laughs. Sometimes used on the supporting characters to make the hero look more competent. It should be noted that this isn't always a bad thing. In some cases, viewers may find the over the top idiocy of a character more entertaining than the character's original concept. Or sometimes, it's just that the show gets a pack of new writers and they turn the guy into a moron for kicks.

Book Dumb characters seem particularly prone to this; it is very easy for writers to go from 'savvy but unintellectual' to 'just stupid'.

May result in a typical Idiot Hero degenerating into Too Dumb to Live. Contrast Took a Level In Badass and Dumbass No More.

Anime and Manga

 * Sailor Venus from Sailor Moon. See Flanderization.
 * Tenchi Muyo!: Mihoshi, especially in Tenchi Universe. Somewhat justified in the case of Tenchi Universe (and Tenchi in Tokyo) since those aren't actually the same Mihoshi, due to those being alternate continuities. Totally unjustified in the 3rd installment of the OVA series, though.
 * Gourry from The Slayers degraded from Idiot Hero to The Ditz and then became even dumber.
 * The degradation arguably occurred before that, considering how Gourry's dumbass swordsman persona was apparently all an act in the original novels.
 * Code Geass: Ohgi, after falling for Villetta. It's so bad that thanks partly to him,, which causes to implement the.
 * Kämpfer: Natsuru but only in the anime. In the manga and light novels however...
 * Ranma ½: Genma. At the beginning of the series, he's lazy, doesn't think things through, and isn't the brightest bulb on the circuit, but is for the most part a competent martial artist who, despite some (admittedly quite serious) blunders, has trained his son to be an incredibly skilled martial artist. As the manga goes on, he becomes increasingly stupid and incompetent, with only one bright spot during the Ryu arc.
 * Death Note:
 * Light Yagami. He goes from erasing his own memory, trusting in the brilliance of his planning to ensure L's death while cementing his own complete innocence... to hiding the Death Note in storage and really hoping no one finds it while he's not looking.

The whole Mello and Near arc has Light nerfed down hard, making great mistakes apparently to balance the odds of the two kids-nemesis. He loses the note, he loses Sayu, he loses his father... and one of his "best" responses is to confirm to Near he's Kira by sending a mass of stupid people at the exact location (and only there) of Near's anti-Kira organisation with the great expectations that they'll lynch someone of L's level. The only vague explanation would be that years as Kami without a Worthy Opponent dulled his senses.
 * That, or it was he was going insane due to his god complex and truly thought that everyone was beneath him.
 * Pokémon: The Team Rocket trio. In the early appearances they are very competent, albeit very hammy, and have succeeded several times in stopping the heroes and capturing Pikachu only to be save by a Big Damn Heroes moment or an Ass Pull of some sorts. Fast forward and their only real purpose now is to show up, provide comic relief, and blast off again, barring the episodes that focus on them being sympathetic to the point where they might as well not be villains if not for status quo getting in the way.
 * This is finally subverted in Best Wishes as they now Took a Level In Badass. Ash, however...
 * Alphonse in the 2003 anime version of Fullmetal Alchemist. He starts off as an intelligent, insightful young boy who was the sensible one compared to Ed. Later on he makes many very poor, and arguably stupid choices like taking Barry the Chopper's mind games to heart, and distrusting people who he's known all his life. This happens in both the manga and anime, but in the manga, Al learns from this and doesn't do anything like it again. In the first anime it mainly takes effect in the later half,where he runs off every time he gets upset,even though the Homunculi  are looking for and trying to kill them, going along with Shou Tucker's plans and not telling Ed (even though Tucker made his daughter, and letting Sloth convince him that she's his mother, even though she is a Homunculi when Ed is trying to remind him that they're fighting for their lives,  He arguably became more impulsive than his brother, whereas earlier in the show it was the other way around. This could be interpreted as the writers having Al act more like how a real little boy would if they were put in such a situation, . Other than that he basically becomes quite the load.
 * As of late, whenever Naruto seems to start growing as a character, he immediately takes a level in dumbass. Most notoriously when he doesn't realize anything is wrong during the attack on turtle island by Kabuto and zombie!Deidara. Despite the living island suddenly acting as if it were hit with a high-level earthquake.
 * Sakura Kinomoto of Cardcaptor Sakura plays with this. In early episodes she was something of a Naive Everygirl, childlike but fairly normal in terms of her age's intelligence and demeanor. In later episodes she is something of a Genius Ditz, highly developed in her handling of her cards and even the odd bit wisdomic, but an exagerratively naive Kindhearted Simpleton in most other areas.

Film

 * Idiocracy: This trope happening to the entire human race is basically the plot.
 * The Neverending Story III: Falkor, who is supposed to be a very smart character. He is a smart character in the first film and still somewhat smart in the second. In the third, however, he becomes a downright moron.
 * Not to mention the Rock Biter...
 * The Mask has both a film-wise and adaptation wise example for Lt. Kellaway's partner, Det. Doyle. In his initial scene at the bank, Doyle seems to be quite intelligent. By the time we see him at the park, he's suddenly a ditz and by the film's end he's a certified idiot. The cartoon made him even dumber.

Literature

 * Sherlock Holmes: Watson is compentent in the originals novel, though his mediocrity does serve to highlight the main character's brilliance. In many adaptations he has taken a level in dumbasss: few of which have portrayed him with any of his original intelligence or abilities.
 * Same goes for Hercule Poirot's sidekick Captain Hastings.
 * Wheel of Time: In the first four books of the series Elaida is presented as a cunning member of the Red Ajah who is able to get Suian removed from power and still her. As soon as she gain powers her I.Q drops considerably, she pisses everyone in the tower off, she makes disastrous decisions that causes Aes Sedai to be captured, and she can't even decide that the Black Ajah exists or not even when it should be obvious after several sisters were murdered and Ter'angreal were stolen. The only possible explanation is she was elevated beyond her abilities (not surprising given she was the pawn of several Black Ajah in getting raised) and cracked after her first few screw ups.
 * Actually, there's another explanation: she was briefly visited by Padan Fain, whose powers include the ability to corrupt people and drive them insane. In one of Fain's points of view, he mentions that he "brushed [her] with what he carried from Aridhol." It's not exactly confirmed, but it is implied.
 * Word of God confirms it. Fain wasn't around Elaida long enough to substantially change her personality, but his presence brought out the worst aspects that were already there and certainly accelerated her deterioration.
 * Alviarin was a member of the Black Ajah and Elaida's Keeper of the Chronicles. She had been bullying Elaida into obeying her orders, even using Compulsion on her to have her obey. Most of Elaida's efforts since being raised are to undermine Alviarin's influence on her, and it is known that Compulsion causes mental instability if it is used multiple times on one person. Both these factors, as well as Fain's ability to corrupt, would swiftly give her the dumbass experience required to level up from cunning to totally incompetent.
 * Also, by the last several books things aren't going Elaida's way at all, and she's starting to crack under the strain. A Villainous Breakdown may not cause you to lose IQ points, but it'll certainly have a negative impact on your being able to use them.
 * Cersei in A Song of Ice and Fire, although YMMV. In book 1 she successfully engineers her husband's death and beats out Ned Stark in the power struggle that follows, and while she's ultimately bested by the Imp in book 2, she's not a pushover. In book 4, though, she surrounds herself with morons and alienates everybody useful as if she's doing it on purpose - Littlefinger aptly describes her as "stumbl[ing] from one idiocy to the next." One interpretation is that she's gotten stupid and arrogant from her success, which matches this trope. But it's also possible that she's no dumber than before, and has simply been promoted to the level of her incompetence, Peter Principle-style. The Seven Kingdoms are not an easy realm to run.
 * It helps that everyone capable of overriding her bad decisions is dead or has left King's Landing by the fourth book.
 * Kalten in The Elenium seems to get a case of this. In the beginning of the trilogy, he is a skilled knight although basically book-dumb, i.e. he is incapable of performing magic because he couldn't learn the Styric language. By the end of the second book, this has turned into him not knowing what a diagonal is. Subverted at the end of the Tamuli where he reveals that he is playing the Obfuscating Stupidity card, since he knows that if people don't take him seriously they will be off-guard around him.
 * Septimus Heap in Septimus Heap, between Syren and Darke. In Syren he the cautious one, in Darke
 * Claudia from The Baby Sitters Club started out as a C-student who had an above average I.Q. but just didn't care about doing well in school. By the end of the series, she was so dumb the reader sometimes wondered how she managed to put on her pants in the morning.

Live Action TV
"Edmund: That Farrow bloke you executed today, are you sure he's dead?
 * Sam Malone in Cheers. He went from earthbound jock to idiot after Diane left.
 * This was uneven. Some episodes he was still portrayed as being reasonably bright, others he was a lecher dominated entirely by his reproductive organs. It went back and forth depending on whether Sam was being a reality check for Rebecca or trying to bed her.
 * Jayne over the course of the short run of the Firefly series seems to go from unsophisticated but frighteningly cunning in the pilot to just plain stupid.
 * Cat from Victorious. She goes from being of relatively normal intelligence in the early parts of Season 1 to a girl who is apparently intelligent enough to just color a "pretty tiger" purple and realize that "that doesn't happen in nature".
 * Robbie also qualifies, as many of the problems the kids get into over several episodes (notably The Great Ping-Pong Scam, How Trina Got In and Locked Up!), are directly his fault. His behavior in dealing with Rex (notably in Wi-Fi In The Sky) also puts him in this category.
 * The Office:
 * Michael Scott for a bit, but they reined it back in.
 * Inverted with Dwight who started the series as a gullible man with almost no common sense and then became more and more skilled and successful as time went on. By seasons six and especially seven he seems smarter than any of the other characters.
 * Kevin Malone. He was never the brightest bulb at Dunder Mifflin, but he showed signs during the first few seasons of just being a little dopey. A savant at the worst. By Season 5, he had lost about 25-50 IQ points. As of Season 8, he is pretty much mentally retarded.
 * Heroes
 * Peter Petrelli after he learns how to use the powers he absorbs.
 * In fact, everyone on Heroes gets this, with nominal good guys Mohinder, Peter and especially Hiro getting the worst of it.
 * The villains too. Adam Monroe goes from being a coldly calculating bad ass with mad samurai skillz to a guy who meekly allows powerless brainless thug Knox to take him to see Arthur Petrelli. Arthur himself goes from an Evil Overlord big on the "You have outlived our usefulness" who was genre savvy enough to take out everyone who could oppose him to being a dumb ass who stands around and does nothing while Sylar has a bullet aimed at him. The German in the graphic novels was a vicious amoral killer who knew how to use his powers in clever and brutal ways but in the show he's an idiot who just does nothing while Knox lumbers over to him and slams his fist through his chest. Candice goes from being a smart and sadistic villainess to being stupid enough to let her guard down around Sylar. Elle has the exact same thing happen to her.
 * Mr. Linderman. He began as a cryptically whispered name that made the most Badass among the cast cower in fear. He was revealed as an Affably Evil Well-Intentioned Extremist Chessmaster who had been engineering events across the entire first season. When DL and Jessica show up, what is his plan?  Yeah, really bright, Linderman.
 * Kelly Bundy from Married... with Children. The more her intelligence decreased, the more the show's quality increased in direct proportion.
 * Hilariously subverted in the episode "Hi, I.Q.," in which she accomplishes what Al couldn't--assembling his workshop bench. Which could just suggest she is simply Book Dumb.
 * Michael Kelso from That 70s Show. He was always a ditz, but he just got dumber and dumber as the show progressed.
 * Chelsea from That's So Raven.
 * London from The Suite Life of Zack and Cody. And again on The Suite Life On Deck.
 * Max from Wizards of Waverly Place
 * Eric from Boy Meets World.
 * Supernatural: This has happened to Dean Winchester, starting from the point that he became a Broken Cutie, Depending on the Writer.
 * Jack O'Neill in Stargate SG-1.
 * Initially he was quite intelligent and a prime example of Colonel Badass. He had tendencies of The Watson, but there's no shame in knowing less about science than the one who blew up a sun, less about ancient cultures than the one who's studied them all his life, or less about alien technology than the one who lived with it. Then the things being explained to him got simpler and more obvious as his jokiness quotient rises. At the nadir, we get "La, la, I can't heeeear you!" when finding out a guest star was in fact someone incredibly dangerous from a previous episode, or later gems like "SHE'S HAVING HER BRAINS SUCKED OUT!" By season five, his role seems to be "make jokes during dead-serious situations while others roll their eyes and make the decisions that would have been O'Neill's responsibility." An episode had the people of the planet of the week talking about how Earth would have sent its best and brightest representative... and then O'Neill enters, making one of the sort of inane comments that had become normal for him. When the idea of the leader of the team being a smart and capable guy is a punchline, you know you have a problem. Later seasons seemed to go back and forth with his intelligence.
 * Of course, the characters who know him best, especially Sam, insist this is just Obfuscating Stupidity.
 * As of Stargate Universe, O'Neill limits his silliness to a couple of one-liners per appearance, and seems to have gotten down to business.
 * Rimmer from Red Dwarf, though how many levels taken seemed to change with every season. The Cat is an even better example having started out as... a cat and ended up with "a brain the size of a grape."
 * Joey from Friends seems to have lost quite a few IQ points over the years, to the point where in the final season he can't tell left from right.
 * Also Erica, the biological mother of Chandler and Monica's twins. When we first meet her she's a perfectly normal, intelligent girl. When she appears later in the season she's so stupid that her IQ is merely half of Forest Gump's.
 * Chrissy on Three's Company was originally mildly ditzy. By the time she left the series one wondered how she managed to dress herself or hold a job.
 * Baldrick from Blackadder -- there's the obvious leap between series one and two (while Blackadder himself does the opposite and takes a level in smartass), but even after that, he does still manage to get gradually worse over the next three series, until in Blackadder Goes Forth he's what Tony Robinson described as "terminally stupid" -- you literally can't imagine how he's survived to adulthood. Of course, in Blackadder Goes Forth his childlike personality is both hilarious & tragically poignant, given the hopelessness of the World War I trench setting. He even gets some moments of childlike wisdom in the finale, simply questioning why they can't all just stop fighting and go home.
 * If you want concrete evidence, compare this scene from Blackadder II...

Baldrick: Well I chopped his head off, that usually does the trick."

"Edmund: [...]So, what's the plan?
 * ...with this one from Blackadder The Third

Baldrick: We do... nothing.

Edmund: Yep, that's another world-beater.

Baldrick: Wait, I haven't finished. We do nothing until our heads have actually been cut off...

Edmund: ...and then we spring into action?

Baldrick: Exactly![...]"

"Mac: Dammit, Charlie, your illiteracy has screwed us again!"
 * The character changes between series are all justified because it is a different Baldrick in each one.
 * Matthew Brock from News Radio is another example; in the show's first season he seemed relatively normal and competent (he even secretly was ), but by the third or fourth season he came across as either mentally retarded or an eight-year old in a man's body. Jimmy James to an extent too, although that was more of Taking a Level in Craziness than Dumbness. Both characters were a lot funnier and more interesting due to these changes, of course, due to the quality of the writing & acting.
 * While Matthew has no excuse, Jimmy James may have been engaging in Obfuscating Stupidity. Alternatively, he really is that eccentric, and his business skills are only used as he needs them. This is apparent given that Mr. James is implied to be a highly successful Self-Made Man, and his occasional bouts of serious behavior back up that he didn't get there by being an idiot. Most likely, he really is a goof ball and we generally only see him when he's relaxing, rather than engaging in Serious Business (which is on several occasions implied to involve questionably legal activity).
 * Radar O'Reilly of M*A*S*H could be regarded as a variation of this. He didn't get dumber, per se, but more childlike and naïve as the series progressed. There's an element of Ping-Pong Naivete there too, however, as he was apparently still savvy enough to make deals for supplies with other units, keep all the paperwork straight, and otherwise serve as Hypercompetent Sidekick to Colonel Blake (and, later, Colonel Potter).
 * Possibly justified. A recurring theme of the series was that people found odd and near-insane ways to cope with what they were experiencing. Radar's steady retreat towards childhood is actually one psychological response people have to stress in the real world.
 * Radar's character regressed in a lot of ways. Early in the series, he was seen smoking cigars, drinking Henry's booze, and had several references to him peeping in the nurses' showers and enjoying developing their chest x-rays. As the show progressed, he got dumber, more naive, and less experienced to the point that even grape Nehi was a strong drink for him.
 * To be fair, grape Nehi is a strong drink for anyone. Bill Bryson solemnly swears that he once saw the edge of the universe after drinking it as a child.
 * Nate Archibald on Gossip Girl. While he wasn't as intelligent as Chuck or Blair in season one he was still a guy of normal intelligence. By season four even his actor is complaining about what a moron he has become.
 * In fairness, he's done so much pot in the last four seasons it actually does make sense that he's dumber.
 * Granted, Potsie Webber of Happy Days started out a little on the dim side, but he only got dumber and dumber and dumber as the seasons went on.
 * Nearly everyone from Charmed but Paige was probably the worst affected, going from one of the best things about Seasons Four and Five to becoming a spazzy idiot. Best example of this? ORB THE FUCKING GUN, RAIGE!
 * Joe from Wings after season five.
 * Charlie on It's Always Sunny in Philadelphia. He was always the least intelligent/educated member of the group, but over the course of the first few seasons he got hit with this trope so hard he lost the ability to read and write (though he seems to think he can).


 * KC from Degrassi, he's introduced as a member of the school's gifted program, two seasons later he thinks there's four trimesters during pregnancy.
 * Randy Disher on Monk seems to get more and more improbably stupid as the series progresses.
 * Robin Scherbatsky on How I Met Your Mother didn't get dumber per se, but she did acquire a lot more quirky traits, gaps in her understanding of American culture, and general insanity (*salutes*) in late season 1 through early season 2. The relative mildness of this effect meant it served to make her character a lot funnier and well-rounded: before, she had been an interesting character, but had a fairly narrow, undeveloped scope. This trope also actually made sense here, as she wasn't really a part of the gang in season 1, when her connection to the other characters was being Lily's new friend and the girl who Ted had a crush on, so she wasn't especially open with them and didn't spend as much time with them as she would later on.

Newspaper Comics

 * Dilbert: The defining character trait of Dilbert's boss was a result of this. In the earlier strips he was just a regular Bad Boss who was more likely to be shown performing evil experiments on his employees than losing at chess to a pineapple. And done deliberately in one strip where a new guy transitions from appearing to be smarter than everyone else in the team to having neanderthal-level intelligence over the course of a three panel comic.
 * FoxTrot: Roger, the father figure. In the earliest years of the strip, he was a competent enough patriarch, though a bit harried and somewhat of a Ted Baxter. As the strip went through Reverse Cerebus Syndrome, his intelligence dropped severely and his occasional bouts of incompetence (he was a Walking Techbane from the first month onward) became exaggerated to match.
 * Garfield: Jon Arbuckle, Garfield's owner. He started as a perfectly ordinary young man. Come the early-2000's, and he's a total moron who finds interest in mundane activities, mistakes a sudoku for a crossword, and mistakes an upside down restaurant menu for French. It was around the same time he was Flanderized into being the Straw Loser to Garfield.
 * Peanuts: Sally Brown started out as reasonably naïve and childish before devolving into a full-on The Ditz who made hilariously inaccurate school reports.

Video Games

 * Final Fantasy: Rikku got significantly ditzier between Final Fantasy X and X-2.
 * Perhaps it was just Kazushige Nojima's shaky writing (he admitted in an interview that he didn't do a lot of work on the first game and wasn't familiar with the plot/characters/cosmology of KH), but Sora went from often naive in Kingdom Hearts to often ridiculously clueless in Kingdom Hearts II.
 * Pete Wheeler of Backyard Sports. I mean, he forgot how to ride a bike within a few months of being a pro.
 * Super Mario Bros. Has Bowser. Originally, just a straight up, Obviously Evil, Designated Villain. The lighthearted nature of the series however, lead to his portrayals as a dumb, immature bully, leading him down the path of this trope.

Webcomics

 * In Sluggy Freelance Sam "Da Man" Sein's intelligence has been plummeting ever since he became a vampire.
 * Eight Bit Theater:
 * Bikke the pirate was actually fairly ruthless and cunning when he first appeared (well, compared to Garland and the heroes, anyway). At this point, his intelligence is viewed as on par with Fighter's. In fact, he's probably worse since he's not even literate.
 * D'rizzl actually inverted this trope, by taking levels in Smartass. Complete with a 50-point IQ hike.
 * Fighter himself could also be considered a victim of this trope. Initially, he was simply The Fool with an intense sword obsession. For the past few hundred comics, he's essentially been a total idiot.
 * Could be justified due to brain damage as a direct result of Black Mage's stabby urges.
 * Nick in Schlock Mercenary, who went from "stereotypical dim-bulb grunt" to an extreme ditz.

Web Original

 * Caboose from Red vs. Blue. This was explained in-universe as the result of the trauma of losing his girlfriend (Sheila the tank) having his mind controlled by O'Mally and not being too bright to start with.
 * Oddly enough, starting from Recreation, Caboose actually seemed to regain some of his intelligence, upgrading from a complete idiot to merely The Ditz. This could be explained by the time Delta spent in Caboose's mind.
 * Homestar Runner: Homestar himself. Which makes his random moments of intelligence all the more hilarious. He once recited Coulomb's Law when asked "What is 2 plus 2?" (when Strong Sad reiterates the question, Homestar naturally gets it wrong), and actually outsmarted Strong Bad by getting him to say something stupid.
 * The reviewers of That Guy With The Glasses, whose characters are often not the smartest or most stable people anyhow, all seem to take stupid pills for the annual Massive Multiplayer Crossover anniversary films and forget about the intelligence, resources or super-powers they've displayed in their own series. This is more or less justified by being a kind of Recursive Canon, where the characters have made their videos instead of living them, but are still those characters instead of the people who actually play them.
 * Happened to both The Nostalgia Critic and The Nostalgia Chick. The Critic used to be of average intelligence before morphing into a ditz with almost no common sense but can very insightful when he wants, and the Chick used to be an Insufferable Genius before becoming a Know-Nothing Know-It-All who is oblivious to most things going on around her. Tropes Are Not Bad of course, their characters seen as much funnier now.

Western Animation
"Frank Grimes: I don't know anything, because I'm Homer Simpson!"
 * Homer Simpson, of The Simpsons, has never been the brightest bulb in the box, but he actually started out a fairly normal person. He just got dumber and dumber as time went on. There have been a couple of in-show explanations for this, all contradictory. Conan O'Brien said on a DVD commentary that Homer's IQ started out at 65 and dropped 5 points a year until it hit zero in Season 13.

"Professor Farnsworth: And, Fry, you've got that brain thing.
 * Ralph Wiggum has suffered from this trope more than any almost any other character mentioned. He was originally just another generic classmate of Lisa's before becoming the unbelievably dense collection of non-sequiturs we all know.
 * Word of God says that Ralph was originally intended to be an eight year old version of Homer. When it they realized that didn't really work, they just made him the most childlike character imaginable.
 * South Park: Almost every single adult undergoes this to a degree, but Randy Marsh is possibly the greatest example of this trope ever. Through the 13 seasons of the show, he has taken more levels in dumbass than any character other than possibly Ralph Wiggum.
 * SpongeBob SquarePants has always been a little loopy. But he has become actually dumber and dumber (not just loopier and loopier) as the series has progressed.
 * Especially after "A Pal For Gary". You'll want him dead by the end.
 * This trope also applies to SpongeBob's best friend Patrick Star. Oh so much...
 * The Fairly Odd Parents: Pretty much every character with the exception of Wanda. Timmy started off simply as a ten year old boy who usually made rash decisions without thinking his wishes through. The newer series makes it debatable how he hasn't destroyed the world already yet.
 * Cosmo most definitely. His first appearance was of a suave, gentlemanly persona in the first episodes, who made sarcastic comments and wisecracks. Now he's a screeching, literally brainless fairy who would have already killed Timmy if not for his wife's constant surveillance.
 * Space Ghost Coast to Coast: Done with several characters.
 * King of the Hill
 * All of Hank Hill's friends seem to slowly lose their precious remaining IQ points over the course of the series -- resulting in dumber and more outrageous schemes that Hank must eventually fix.
 * Luanne and Peggy also. Peggy has never been the Closer to Earth person between her and Hank, but she used to be at least a competent Spanish teacher -- now she's shown to have a tenuous grasp at best on the Spanish language, though it does lead to some admittedly hilarious moments. Luanne has had a similar path of regression: after an angel tells her to go to college, she marries a Creator's Pet. This may have had something to do with KOTH being replaced by The Cleveland Show.
 * Peggy was bad at Spanish at the beginning of the series, in fact, she actually got a little better as it went on. The rest here is spot on, though.
 * This happened to Fry in Futurama. In the earliest episodes, where he was supposed to be a Fish Out of Temporal Water and stories would mostly focus around him adapting to life in the 31st century, he was a fairly average guy with a dash of Cloudcuckoolander and a few moments of genuine cleverness. As the show progressed, he adapted to his surroundings much faster than the writers intended, and so he devolved into a gibbering manchild unless the writers needed him to say something profound.
 * It's somewhat justified in-universe with Fry's lack of a delta brain-wave, which effectively stunted his mental growth:

Fry: I already did!"

"Fuzzy: Come back here an' git off mah proppity!!"
 * Of course, the means in which he contracted it — as shown in the episode "Roswell That Ends Well" — would have meant Also of note is that, in "The Why of Fry",
 * Johnny Bravo: Johnny. Ironically, when the series was temporarily revived, Johnny wasn't fucking retarded, instead trying to avoid his now-annoying neighbor, Suzie.
 * Family Guy: Peter Griffin was a dumbass to begin with, but it got worse with uncancellation. Now he's gotten even worse. He's legally retarded in-canon. You never would have seen that coming in the show's early seasons.
 * Back to The Future The Animated Series: Marty McFly. He went from Book Dumb in the films to The Ditz in the series.
 * Chowder: Chowder; compare the naive little boy of Season One to the completely Too Dumb to Live of Season Two.
 * Beavis and Butthead started out as just stereotypically lazy and not all that bright teenagers. As the show's popularity picked up the need for more outrageous material resulted in them becoming the Too Dumb to Live types everyone knows them as. Beavis especially went from a generic dumb teenager to a bizarrely retarded boy with a split personality.
 * There was an episode of Re Boot, where Enzo wished to be smarter than everyone else. So the system underclocked everyone else, making them stupider so that Enzo would be the smartest person in Mainframe.
 * In season one of Frisky Dingo, the villain Killface is intelligent, educated and sophisticated, by contrast to the Idiot Hero Awesome X. A lot of the comedy from Killface stems from the contrast between their personae. Come season two, they're both complete morons.
 * The Powerpuff Girls: Fuzzy Lumpkins took this between the pilot and the series proper: in the pilot he was a reasonably calm furry... thing with enough intelligence to build a gun that turns objects into meat; in the series he's a trigger happy hillbilly stereotype whose idea of a weapon is a rock (compared to, say, the ray guns of Mojo Jojo and Princess). Consider his line in "Fuzzy Logic" while chasing a squirrel from his home:


 * Daffy Duck for The Looney Tunes Show. In the original shorts he was fairly intelligent and clever, but occasionally gullible and was often defeated due to his greed and selfishness. Here, he's a lazy idiot who can't do anything right and fails to answer even the simplest trivia questions correctly.
 * Penfold on Danger Mouse was the token dumbass, but as the series wound down to its final two seasons, Colonel K was afflicted by a case of the dumbasses which could be chalked up to early senility.
 * The Battletoads cartoon turned the three protagonists into brain-dead teenage idiots. It even introduces Morgan "Zitz" Ziegler, a Genius Bruiser in the game canon, by having him cause a computer to explode just by trying to type something into it.
 * Cleveland grew progressively dumber when he left Family Guy and starred in his own show, The Cleveland Show. Cleveland used to be the voice of reason in Peter's group but once he moved away, he became nearly incompetent and childlike.
 * T.J. from Recess started out relatively average in the first season. He became more Book Dumb in season two, and by season four, he was The Ditz.