To what extent does Harper Lee present Maycomb as a Microcosm of American society in the 1930's?

I believe Lee was presenting a problem to the public eye that needed remedy. I think she was in a sense challenging the status quo. She presented racism in a realistic light and exposed it as a moral flaw. Yet she expressed no hatred or animosity toward any person in the book with the exception of the plaintiff's father. The rest of the characters are shown, by the author, either pity for their ignorance or economic problems or respect and admiration for using wisdom and intelligence to attempt (even if one fails) to instigate social change. Education is god in To Kill a Mockingbird. Atticus Finch was a lawyer of great intelligence who was the hero of the story. Scout was his daughter and we see her education from her father to teach her that all humans are created equal in the world and we are to use our strengths (whether physical or intellectual) to protect the ignorant and weak not to bully and intimidate.
So yes I believe Harper is calling for change in the world. If she was merely describing life as it is in Maycomb her book would be nothing more than nihilistic resentment. It is much more than that.