I saw Travis Bickle as far more sympathetic, that even though he was a racist weirdo, he didn't really have bad intentions .. i guess except for the senator, but he didn't actually do anything. pretty much all his violence was in service of saving Iris.

i suppose Arthur was symathetic in the sense that he wanted people to smile and laugh, and he doesn't start the movie as a violent guy but he feels circumstances drove him to where he is. he gets shit on, made fun of by his peers and society, his "dad" rejected him, his mom lied to him, Deniro talk show host used him to mock him. I can see the 4chan/incel type crowd feeling he was justified on some fuck everyone, but I don't see him as all that sympathetic to your average viewer, and it's sealed at the end as you mentioned with the doctor.

i loved the movie though