For what I remember, Mabel needs some Unicorn hair for an Anti-Bill Cipher thing.
The Unicorn doesn't give it to her because she isnt "pure" enough, leading Mabel to do a bucket list containing good deeds or something.
The Unicorn is never satisfied, and Mabel gets sad. The twist however, is that the Unicorn was actually only being a dick, and when the girls notice they beat the shit out of it and steal the hair.
The morale is don't attempt to be perfect???
It wasnt a bad episode but, considering it was one of the final ones, it came off as just, eeh? I would have loved it if this situation actually developed more on Mabel's insecurities instead of the conclusion being "Unicorns are assholes lmao", considering what Mabel does in the finale, some foreshadowing would have been good.
It's better than Roadside Attraction though, that one was shit.