What are some ways that Tom Robinson in To Kill a Mockingbird is not a "mockingbird"? Please provide some points to help prove this opinion.

Before answering this question, it's important to understand Harper Lee's use of the mockingbird symbol in To Kill a Mockingbird. Early on in the novel, Atticus asserts "it's a sin to kill a mockingbird," and we go on to learn about the beauty of a mockingbird's song and the purity of its intentions. As such, the mockingbird is generally seen as representing innocence, and so the act of killing a mockingbird can be seen as a loss of innocence.

This loss of innocence is central to the novel, and it is possible to view Tom Robinson as a personification of a mockingbird, as he is innocent of the crimes he's charged with and is a virtuous person chiefly concerned with helping others, working hard, and providing for his family. However, it's important to avoid getting too wrapped up in this association, as there are many ways in which Tom Robinson is not a mockingbird. 

While Tom may be innocent of the crimes he's charged with, he does not possess the naive innocence that a mockingbird symbolizes. After all, Tom is a fully-grown adult, has already experienced the hardships of adult labor (as evidenced by his mangled arm), and is fully aware of the systemic racism that oppresses him (as evidenced by his decision to flee from the Ewell house for fear of being charged with rape). Indeed, Tom's fatal attempt to escape from prison illustrates his knowledge of society: he knows he has no hope of freedom as a black man even though he committed no crime, and so he desperately tries to take matters into his own hands. All in all, Tom is not "innocent" in the naive, childlike sense, since he is actually very experienced. He has had plenty of exposure to the harsh realities of the world he lives in.

Indeed, the idea of killing a mockingbird more effectively parallels the growth of Jem and Scout (and Dill, of course). Witnessing the racism, classism, and sexism at work in Maycomb wrenches the three characters out of their childhood innocence, forcing them to confront the unsavory adult world. As such, while Tom Robinson can be seen as a partial representation of a mockingbird, his well-established experience in the adult world prevents one from reading him as a representation of child-like innocence. That designation is better suited to the children, especially Scout, at the heart of the novel.