I've said elsewhere that Joker stories should NOT be about the nature of mental illness. I don't like (comic book) stories that make mental illness or emotional problems look like some sort of moral failing the person can't "get over." Peter David's FNSM #5 "Web Blog" had this problem.
And this story seems to be just that. "Killing Joke" at first glance seems to have this problem as well, but Batman points out to the Joker "Maybe the problem isn't the whole world drive people crazy; maybe it's just *you*."