Why is it that people always think good villains should always be completely dark and scary without any unique gimmicks? I don't see anything wrong with a clownish personality as long as they're still a deadly threat. The Joker is a good example of this, so why is he always excused from the equation?
Because there's more to the Joker than that. While he is constantly wacky (that's sort of his gig), he is frighteningly evil and ruthless. I'd wager to say he's the darkest Batman villain there is. And he's made all the more terrifying by the fact that he clearly takes extreme pleasure in every second he is causing someone anguish. I can't say I've ever seen Eggman demonstrate that kind of sadism.
The closest he came to that was during Sonic Adventure 1 and 2, especially in 2 where he was arguably at the most evil he'd ever be within the games themselves. Aside from those times... well, it's not a very encouraging sign that he apparrantly needed an evil twin. Let that sink in; he needed an evil twin. Someone explicitly crueller and more ruthless than he. When you're the principle bad guy of a series and this happens to you, something has gone horribly awry. Nor does it help matters that any of the logical consequences of his actions over the course of those games do not really come up or amount to anything; heck, the moon in subsequent games is perfectly intact.
Heck, to take the analogy closer to home then the Joker, take a look at Pokey (Porky?? Never sure which of those to use) Minch from the Earthbound/Mother series of games... he's a spoiled little fat boy, and yet over the course of those games he establishes that he's utterly evil, and despite never losing that bit of goofiness inherint in his character he's STILL an unnerving and frightening villain.
