Does Junior become a better person at the end of the novel?

Junior is a better person by the end of the novel, in my view. He is able to reconcile with his old friend Rowdy, and he tries to build up Rowdy's confidence. When Rowdy says he'll never be able to afford air conditioning, Junior reminds him that he (Rowdy) will likely be a professional basketball player and that he's the fastest, strongest, and best-looking kid on the reservation. Junior is able to be sympathetic to Rowdy because he knows that Rowdy's remarks often come from a place of insecurity.
Junior is also able to face his own insecurities and fears. When Rowdy challenges him to climb the biggest tree on the reservation together, Junior has to accept. Though he is afraid, Junior manages to get to the top of the tree, where he and Rowdy admire their view of the world. Junior pushes himself to climb the tree because he is more confident and because he wants to honor his friendship with Rowdy.
In the end, Junior also tries to forgive himself for leaving the reservation and for his desire to find something different. Rowdy refers to Junior's ways as "nomadic," and this term helps Junior accept himself and hope that others on the reservation will forgive him. By the end of the novel, Junior is in many ways a better, more mature, and more self-accepting person, though you may have a different take on the matter.