Tbh I don't think villains exist in the DC Superhero Girls 'verse... I'm half-expecting that when/if Joker shows up, he'll just be a boisterous class clown who needs a sensitivity lesson or two.
Villains do exist in the universe, though a lot of them are more gray than actual evil. I know Killer Croc is a grade A bad guy while Riddler has only broken some laws (for love). Cheetah is, I guess a good person, but she’s a main source of tension to others.
I haven’t seen the full series yet so I could be wrong, but Joker has never been shown or mentioned in this universe. And I think it’s a great idea because Joker doesn’t need to exist in every reality.