Oh man, comics labouring the point of how insane the Joker is is one of my biggest pet peeves. Because he's not. Most of the time he's not written as even mildly psychotic.

If writers really wanted to give him some sort of realistic diagnosis, they should go for Antisocial personality disorder. And really, even that is more an uneasily medicalised description of a particularly baffling and objectionable type of person rather than a clear cut 'condition.'