Very appropriate. Tom is murdered in this book. He is a mockingbird...one who is beautiful and sings a lovely song of humanity and what we can be if we overlook social constraints in order to do the right thing.
There are other mockingbirds in the book, as well. Boo Radley, for one.
Both the characters of Jem and Scout can be related to the title as well. If you think of growing up as metaphorical shooting, Jem and Scout have has some harsh realities to deal with during this book. Both grow up and experience some disillusionment, but when Boo Radley not only comes out of his house, but saves their lives, he allows them to be kids again--to believe in the goodness of their neighbors and the safety of their neighborhood.
Jem and Scout are two children who never did anything, and Bob Ewell wanted to kill them; Boo prevented the "senseless slaughter of songbirds" and became a mockingbird himself.
The conversation in which Atticus tells Jem and Scout why it is a sin to kill a mockingbird (because they don't hurt anything, and all they do is sing for us) is a major indicator as to why Harper Lee decided to coin her novel To Kill A Mockingbird.
Both major storylines, the trial of Tom Robinson and the mystery surrounding Boo Radley, can be related back to this title. Tom is completely innocent; he's definitely not raped Mayella, and most of the town seems to believe him. However, he is still convicted and eventually dies. Tom, like the mockingbird, never did anything to hurt anyone. In fact, we find out during the trial that he actually feels sorry for Mayella. In this instance, the "mockingbird" (or Tom) is killed -- literally, yes, but also figuratively as well because his innocence is taken from him.
Like Tom, Boo does nothing to hurt anyone. At the end of the book he even rescued Jem and Scout from a drunken Mr. Ewell. Instead of allowing him to be tried for this murder, Atticus and the sherrif of Maycomb decide that it's just not necessary. Even though he'd be found innocent, to be put in the spotlight like that would be like killing a mockingbird -- taking away a piece of Boo's innocence. In this instance, the "mockingbird" is saved.