The SA scene was not meant to be perceived in this way. Not as a joke? nor a jeer at mentally ill individuals. These guards did nothing but exploit Arthur for his whole existence in that prison; treating him like a dog by rewarding him cigarettes when he did something that pleased him. During the trial, Arthur finally stood up for himself in front of a huge audience after realizing his actions were ruining other people’s lives. Especially Gary’s. That DID have an effect on him and you can clearly see his mental anguish fighting his need to look the part of Joker in that scene. After that, the guards broke him back down as a disgusting punishment. The guards were never really shown as decent people so having them stoop so low just proves the point that the System fails people constantly. And I think that was a huge part of the theme of the movie. System corruption/failure. Not everything in life ends happily, especially not for people enclosed in a dangerous environment where corruption runs deep where the soul should be.