How did Mr. Underwood, in his editorial on Tom Robinson's death, use the symbol of the mockingbird?

Mr. Underwood does not explicitly use the word "mockingbird" but he does say that Tom's death is like what happens when hunters and kids kill "songbirds" for no reason at all.

What Underwood is saying is that the people with power are hurting those who have no power (like a person shooting a bird).  Moreover, he is saying that those with power are doing this for no good reason -- just because they can -- just as there is no good reason to shoot a songbird because you can't eat it.

So Underwood is arguing that some whites are simply abusing their power because they can.