InTo Kill a Mockingbird, Tom and Boo are some of the most innocent characters readers tend to sympathize with. But given Miss Maudie's definition of a mockingbird, there are other characters who fit the description.
Mockingbirds don't do one thing but make music for us to enjoy. They don't eat up people's gardens, don't nest in corncribs, they don't do one thing but sing their hearts out for us. (Chapter 10)
Atticus is the paradigm of honesty, morality, and integrity in the novel. He's chosen to represent Tom because he is the only one who would willingly accept the job and the only one to do an honest job of it. When it comes to Tom, Mrs. Dubose, Boo Radley, or his children, Atticus does what he thinks is best according to his moral code and that code is based on principles of generosity and honesty. He only helps people. Consider the episode in Chapter 23 where Atticus describes his encounter with Bob Ewell. Ewell spit on him and challenged him to fight. Atticus just took the abuse in the hopes that it might save Mayella a beating.
One might even compare his defense of Tom, his actual voice, as the mockingbird's song: a mockingbird singing for/defending another mockingbird.