Actually, they both are mockingbirds. All Tom Robinson ever did was help Mayella Ewell and she turned around and caused his death. All Boo ever did was reach out in friendship to the children and protect them in their direst need. Tate even says, “There’s a black man dead for no reason…” He realizes Tom’s death was unjust. Tom is the mockingbird who gets shot (6 times IIRC) and Boo is the mockingbird who lives.
I agree that they both are the “mockingbird”.