What theme or lesson do you think Scott Westerfeld is trying to teach the reader?

Initially, Westerfield presents beauty as a fixed quality in the world of Uglies, referring solely to physical appearance. Those who have undergone the operation are pretties, those who have not are uglies. Those who do not undergo the surgery have no shot at living a successful and happy life, because society will ostracize them. Consequently, uglies grow up with severely damaged self-esteem, unable to see their bodies in a positive light. But Tally's experience in the Smoke, and most especially her romance with David, complicate her understanding of beauty. She realizes that it is a relative and subjective term, applicable to a person's inner as well as their outer self. Accordingly, there are many ways to be beautiful, and diversity is what allows beauty to flourish.