Interesting statement you have to discuss there. I am not too sure that I agree with it if I am honest, because whilst the theme you identify is definitely present in this tremendous short story, I don't actually believe that it is the principal theme. The only example in the story we are given of humans being fundamentally cruel to others is the young couple towards the end who insult Miss Brill, calling her "an old mug" and describing her much-prized fur as "fried whiting." It is this overheard conversation that of course pricks the bubble of Miss Brill's glorious "drama" that she considers this scene to be, and forces her to confront what is a much bigger theme by far: her own loneliness and isolation.
So, whilst I would agree that the indifference of human beings towards others is something that is evident in this story, because after all part of Miss Brill's elaborate fantasy is that "somebody would have noticed if she hadn't been there," by far the main theme is the tremendous isolation and loneliness as experienced by one female character who is only able to live life as a spectator sport by enjoying the lives that others lead instead of living life herself.