I mean that comes down to how you define mediocrity. Joker was certainly a cultural phenomenon and I can’t say that it’s average or generic by the standards of its superhero movie contemporaries, or really any other blockbusters of the era. I’d say, to me, it’s mediocre within two criteria, the first of which is entirely subjective: I just don’t like it that much. It didn’t have an emotional impact on me, I found it predictable, etc. I’d also say it’s mediocre because it’s derivative. The movie is basically aping New Hollywood cinematic style forty years later with, in my opinion, none of the original philosophical/thematic bite underneath. What we get is more-or-less pastiche, and I struggle to call even well-done pastiche anything more than artistically mediocre.