Harper Lee presents American society in a couple of different ways in To Kill a Mockingbird. First, and most obviously, Lee presents America as a society rife with racial tension, division, and prejudice. Though it's clear that Tom Robinson is innocent of the crime he's accused of, he's still found guilty in a court of law. By making this plot point central to the story, Lee illustrates just how much racism is present in American society, as she points out that racial prejudice has significantly corrupted the American legal system.
To make matters worse, Lee also suggests that American society is crippled by a faulty public education system. In her exploration of Maycomb's public schools, Lee paints a picture of a stifling, dysfunctional atmosphere that rigidly adheres to curriculum, even if doing so means stunting the intellectual growth of gifted students like Scout. As such, it would appear that, not only is the United States riddled with prejudice, it's also hampered by an education system that promotes, rather than eliminates, ignorance.
There are, of course, many exceptions to these rules. Atticus proves to be the book's moral compass, and he is both highly educated and principled. Other characters, like Miss Maudie, exhibit similar characteristics. That said, Lee portrays these characters as exceptions to the rule. As such, much of Lee's novel seems to be a critique of American society, especially when it comes to the topics of race, prejudice, and education.