A joker demon works because it subverts the expectation that villains have a coherent, understandable goal. Think about it—most antagonists want power, revenge, control, something you can get your head around. But a joker's motivation is often just chaos or amusement. That lack of a predictable endgame means they can pivot on a dime, ally with the hero one chapter and burn down an orphanage the next, all because it 'seemed fun.' It makes them impossible to outmaneuver through conventional strategy. The hero can't appeal to their better nature or negotiate because there isn't one to appeal to.
I sometimes find it frustrating as a reader, though. If the villain's actions are truly random, it can feel like the author is just throwing curveballs for shock value rather than building a meaningful conflict. The best ones, like Hisoka from 'Hunter x Hunter' or the Joker himself, have a twisted internal logic—a perverse fascination with a specific person or a desire to prove a point about the world's fragility. That gives their chaos a chilling focus.