The man regularly outwits far more supposedly cognizant opponents including Batman and Lex Luthor, who are canonically recognized as two of thr smartest people on the entire planet.
Edit: I’m not saying insane people are stupid, I’m saying that Joker’s mental illnesses are pretty obviously behavioral, they don’t affect his perception of reality. He’s perfectly capable of understanding what he’s doing and how it’s wrong, in fact his character almost doesn’t work if he doesn’t, he just thinks that it’s all hilarious anyways. That’s why I said he’s not insane, he just pretends to be, because he’d be fit to stand criminal trial as fully competent and cognizant of his actions.
Intelligence and sanity are not linked. There are lots and lots of real world examples of very insane people being very very intelligent. There’s also lots and lots of in universe content that shows he really is quite insane. The sheer number of times Joker’s only goal was to get Batman to kill him should be enough proof of that.
My point is that he doesn’t have a warped perception of reality (in the diagnosable sense), being criminally insane would require that he be incapable of understanding what he’s doing and/or the reasons why doing those things are wrong.
Joker not only is perfectly capable of understanding he’s doing wrong and why the things he’s doing are wrong, to him that’s the entire fun of the joke.
He isn’t trying to get batman to kill him because he thinks being murdered sets someone free in any sense like Victor Szazz, he’s trying to get batman to kill him because he believes it’d vindicate his view that all it takes is a single bad day for someone to become the absolute worst humanity has to offer, and because he wants to leave a permanent stain on Batman as a parting FU.
Pretty sure killing yourself to prove a point requires insanity.