Ah, good question, but not one with an easy or final answer. By that I mean, this is a judgment call involving interpretation. You must build a case involving evidence.
There is strong evidence that Tom should be considered a mockingbird. Think of how Atticus says it is a sin to kill a mockingbird—and then the town kills Tom, in what is clearly a sinful act. That's strong evidence, as is Mr. Underwood's editorial. Tom should definitely be considered a mockingbird, and most explicitly. He's the mockingbird who got killed.
However, think of how often Atticus tried to protect Boo, and how special he was once they finally met him. I'd say that he too could be considered one.