Yes, the movie is very realistic, a lonely KHHV who is mentally ill will never be an anarchist revolutionary leader, he is just a poor lumpenproletarian devil who acted impulsively in the face of a society that marginalized him all his life. Even Karl Marx considered these people useless to overthrow capitalism and social stratification.
Many retarded people complain about the rape scene, that scene has a deeper meaning and it is that Arthur not even being the Joker has any kind of respect because in the end he is still a poor devil who cannot overthrow the segregationist system
Maybe the guards should have just beaten him and not raped him? Yes, but that doesn't take away from the main plot of the film, it's just a detail, and it's not something unrealistic because it's something that, although it's not common, can happen in prison. In any case, it indicates how Arthur continues to be humiliated by society.
Finally, it's mentioned that Arthur never had a girlfriend, and his first girlfriend is Harley. Do you ascend at the age of 30-40? Do you ascend with a roastie that in the end abandons you because she doesn't value what you really are? That's similar to having sex with a prostitute, it's not an ascension. Many escortmaxxers receive something better from women than Arthur did.
In the end, Arthur dies as an incel, that is, as a miserable being without any value and forgotten by everyone.