People say the Joker is Batman’s nemesis, and they’re wrong. Joker doesn’t matter to Batman, he’s just another two-bit villain. But Batman is the Joker’s nemesis. Because the Joker needs everyone to think he’s funny, and Batman is a man with no sense of humour. Batman will never laugh and Joker will never stop trying to get a laugh from him. Batman is a great character to add to any Joker story.
But Joker is way more interesting in stories where he sucks. Harley Quinn is great because Joker is just another rapey man (who looks like Elon Musk) and he only matters because the patriarchy (and Harley) lets him matter. And Joker in Batman Beyond is hilarious, because Terry as Batman does have a sense of humour, and he knows Joker isn’t funny.
also not mentally sound at all himself. he gets traumatized as a child and starts dressing like a bat calling himself the dark knight or whatever the fuck he does. pretty much his entire rogues gallery are mentally ill people, he never puts an end to anything, never make any of them get better, never gets rid of any of them for good, just beats them up for fun and terrorizes two bit street criminals. i don’t even know what really makes him different from any of the arkham asylum escapees.