The movie is from Arthur's POV and there are many MANY points in the movie where you can see that he has a very shallow view of success. If he gets a gf, he'll be happy (but it doesn't really matter what she's like as a person as long as she accepts him), if he can do stand up and make people laugh, he'll be happy (but we never see the audience after his fantasy nor hear his imagined jokes because he doesn't care, as long as he achieves his purpose), if he can go to deNiro's show, he'll be cheered by the public and embraced by the host as a friend and he'll be happy (but it doesn't matter what happens after, if he can make something of himself after that because he just wants that one high). Arthur is the main character, not Sophie, not his mother, not de Niro, not the Waynes'. It's a deep character piece, very unreliable one at that, and there is no need to develope the rest of the characters more than is needed because we're already seeing that Arthur is not reliable himself, so we can also guess there's more to the story than we see. So if the author is complaining that Sophie was nothing more than a prop, then he is right, but he also doesn't get that she was also that for Arthur.