LazyTown/WMG

Ziggy is a younger version of Captain K'nuckles from The Marvelous Misadventures of Flapjack.
Think about it. They both love candy to the point of obsession. My guess is that Ziggy listened to "You Are a Pirate" so many times he decided to become one. Then he moved to Storm-along where he met Flapjack and Bubbie. Oh, and then he turned blue.

Stingy is Tim Langdell as a child.
They are both obsessed about owning things they do not, and have no right to, own. Everyone ignores both of them.

This is an Alternate Universe version of Kirby
We have a pink haired protagonist who is known to like cake. Kirby is a pink protagonist who once went on a Roaring Rampage of Revenge over a slice of cake. Robbie Rotten lives apart from the town and, despite being considered the laziest person in it, does some pretty complex plans. Dedede lives in a castle away from the others, is a lazy glutton, and yet somehow (especially in the anime) carries out some pretty complicated plans as well. Finally Meta Knight once tried to take over the world via an airship, to make it less lazy. Sporticus arrived in an airship, to make the town less lazy. At first I said it as a joke, then it just got kind of creepy.

Robbie is an author.
This could be considered a possible motive for his trying to make the town lazy. If everybody spends their time being active, they won't have time to sit down and read a book--specifically, one of his. So naturally, since Sportacus arrived, his works haven't been selling too well.

LazyTown Takes Place in the future......
where puppets have Taken Over the world!

Stephenie's father sent her to Lazy Town because it's the only place that tolerates humans. Robbie wants to get rid of Stephenie and Sportacus because he likes to be the only human in a town full of puppets.
 * Oh god.

Stephanie is actually dead.
The train she took to Lazytown crashed. The whole series is her dying dream (as she lies in a coma), and the final episode where she gets to go in a plane to a dancing school outside Lazytown  was her briefly regaining consciousness, then slipping back under before her brain shuts down completely. Robbie Rotten's constant encouraging her to be lazy was not an attempt to get her to refrain from moving, but to keep her from traveling to the afterlife (the sunlight from outside where they play could represent the light you're not supposed to go into, I guess?). Lazytown Extra is the afterlife.

Robbie is Belphegor
No, not that Belphegor. Robbie is the manifestation of one of the Stakes of Purgatory, Belphie. Belphie, despite being the sin of sloth is very hard working and dedicated -- because if she does all the work, no one else has to. Robbie works in the same way - he's constantly active in an attempt to force the Sin of Sloth on the residents of the town.

You are a pirate!
Oh, come on.

Sportacus is diabetic
It makes sense right? How else does he get sick RIGHT after he eats anything with sugar in it? Diabetes + super speed metabolism, that's how. I mean, the other characters react pretty much normally to sugar (for TV characters anyway.)
 * Glad to see that there's another person who thinks so.

The writers were fully aware of the double meaning of "You Are a Pirate"
It was written in our current Internet age in ICELAND. "Do what you want 'cause a pirate is free", and references to the locks of a treasure chest could refer to DRM and cracking it.

Sportacus is an Time Lord.
Yes, I said it, a Time Lord. He regenerated from Number 9 a while back and somehow managed to get lost, which is probably why we haven't seen him yet in the series, AND why number 10 took so long to get back to Lazy Town.
 * OR 9 went to go fight the Time War and survived because he regenerated into a mostly human form (ala John Smith). The airship is his TARDIS (the colouring fits, after all).

Alternatively, Nine IS Sportacus' dad.
It's just a matter of why such a massive gap between the two being in Lazytown is another thing all together.

Alternatively, it's 11 in a coma
sportacus just HAS to be the doctor. The only problem is that the show is definitely not in the Whoniverse. Therefore 11 got knocked on his head and is now dreaming he's Sportacus, Stephanie is a companion, and Robbie is a reincarnation of the Doctor's darkside: the Dream Lord or the Valeyard.
 * Or the Master.

Stephanie grows up to be Lady Gaga.
Coloured wig? Techno dance routines? Lots of inspiration from Robbie for her costumes?
 * Not to mention that Lady Gaga's real name is Stephanie Germanotta.

The whole show is the product of a kid who lives in the Unreal universes imagination.
The show uses the Unreal engine in its production, so they might have worked that in to the plot. The whole Unrealverse has extreme amounts of Gore and violence, and the average child would probally be disturbed by it all. Thus, his mind imagines the whole show as a mental safe havern from the tournament and and the war with the Skaarj.

In the world of Lazytown, being lazy slowly turns you into a puppet-character.
The three most active characters are all humans. The lazy ones are all puppets. Taking this further...

Robbie needs the humans to all be puppets for some reason
This is why he needs everyone to be lazy. Maybe being a puppet is symbolic of being under control.