Bwa. Bwaha. Bwahaha! That would be awesome!
If you could get it right, though. Giant robots, battle armadas, Giant Brain Parasites - this is not the kind of thing a d20 kernel is happy with in its native form. It would be entirely possible, of course. Hm...
And I'm not sure I'd label Zim an incompetent. He's probably the second most-aware entity in the Universe, after Dib the Large-Headed. I mean, would you trust the Tallest to know which end of a donut is which? (That's not even a trick question.) And the average human?
One thing's for sure. PCs would have definite advantages over NPCs in such a setting. Plus you might have to generate adventures randomly. "PC #3 accidentally awakens a hidden space vessel buried beneath the Skool; he is assimilated and starts seeking out... jelly? Whatever, I've got a mecha this time."
Oh, and you cannot make this anything but an adversarial game. Almost missing the point of an RPG, but so long as things are kept non-lethal it's OK. Maybe another random 'DM fiat' table to determine just how Dib keeps surviving murderous Irkon technological assaults?