The idea of "the mockingbird" is presented by Atticus when he tells his children to never kill a mockingbird because they only sing and never harm anyone. Boo is potentially the best example of a mockingbird, because he never harms anyone, except Bob Ewell. He only hurts Bob to protect the children. He is viewed by others in the community as a treat, but he is really innocent, almost child-like. Some may say that Tom is also a "mockingbird" because he is trying to help Ewell's daughter and is convicted without any evidence. In his effort to help, he is "killed," much like the innocent mockingbird. Although he is killed trying to escape, he is only in prison because of a false conviction. Many literary experts have also drawn a parallel to the fact that he is climbing over a fence, when he is killed, and the mockingbird would be in a tree.
No, Boo is only one example (perhaps the best example) of a "mockingbird" in the text. Tom Robinson can also be considered a mockingbird, as he does nothing but good things for others, even though his good intentions lead, in part, to his death. One could also argue that Atticus is a mockingbird for his dedication to fighting for Tom's innocence in the face of adversity.