An insane Joker would be totally wrong. He would not be evil, he would just be sick (in the sense of "having a disease"). But the Joker knows social norms but he just doesn't care. That's the point. An insane guy would not be a really, let's call it, enduring villains, because he would not have a clear mind. And I think the Joker's mind is very clear, because if not he would not be the mastermind he is. Yeah, I know, the Joker has been portrayed this way in recent times... but to me the Joker is and was sane, as he was the most time of his fictional existence. That is what makes him devil-ish, insanity would make him just a poor victim.