The mirror at the beginning of Walter Dean Myers' Monster symbolizes a couple different things. First, given the numerous names scratched into it, the mirror represents a history of the men, and boys, who have spent time in the same cell Steve Harmon is locked up in. Second, the mirror represents what Steve has become. Since in jail (arrested for murder), Steve has changed dramatically. When looking into the mirror in his cell, Steve does not recognize the person he has become (illustrated by the fact that he states his reflection "does not look like me"). The mirror, then, shows Steve what others sees him to be (something which he and his lawyer will struggle with over the course of the trial), not what he knows himself to be.
This illustrates the idea that people are not always what they seem to be upon first glance. Steve, as part of his defence, must prove that he is not the typical "young, black" kid seen in court. Unfortunately, the mirror shows this to be true for him. In regards to the novel's theme, the image of the mirror, and Steve's reflection, speak to identity and stereotypes.