A bunch of Batman's villains actually do get better eventually. But then then they relapse.
Clayface was actually on Batman's team for a significant Detective Comics run
Iirc TwoFace was cured in the "One Year Later" storyline following infinite Crisis only to go "lmao I'm two-faced again."
Riddler also spent some time as a good-alligned PI but i forget if he was really good or if it was 4d chess >_>