Gregory
International Sex Symbol
Yes, not interesting. At least, not as you're proposing to use him.
The Joker can be interesting, I think, but the part that I find interesting is the part you are proposing to eliminate: Joker's relationship with Batman. It may be more complex then any hero-villian relationship. It often seems to me that the Joker defines himself in terms of the Batman. In The Dark Knight Returns, it is the news of Batman's return that makes the Joker go active. In No Man's Land, it is clear that the Joker is desperately courting Batman's attention. And in The Joker's Apprentice, Joker declares that only Batman and Joker exist; everything else is just there to pass the time between their meetings (he also declairs that only the night loves Batman more than the Joker). My point, I guess, is that with the Batman, you have an interesting (and probably totally unique) hero-villian relationship. With the Joker by himself, you have ... well, you have a murderous clown. Prime slasher material, but interesting? Not in my book.
Incidently, I've read The Killing Joke. The Joker admits that he doesn't really remember what caused him to go insane; what he presents is only one version.