I don’t think Moore intended for there to be a meaaage regarding mental illness at least.

One of the reasons why he’s pooh-poohed TKJ is he says it doesn’t have a greater message beyond “Batman and Joker are more similar than they’d like to admit” which according to him is only profound to Batman fans and meaningless to anyone else. To me that’s kind of a silly criticism, a story about the Batman and Joker doesn’t need to have something new to say on the human condition but he’s entitled to his opinion.