A mockingbird is a plain, ordinary bird with an extraordinary gift. It can mimic the song of many different birds. It is not a destructive bird like a wood pecker. It is not a predator. Its beauty is its song.
Tom Robinson is an ordinary black man of the time. Yet his kindness and compassion for another human being is his beauty. Even though he has a withered arm that is useless for hard work, he is a good, hard worker. He cares for his wife and children and provides a living for them at no expense to anyone else. His acknowledgement of Mayella Ewell's presence is a kindness. No one in her family cares for her, nor does any self-respecting white person, for the Ewells are white trash that live near the dump and pick through garbage for a living. Tom notices her pride in her plants and her cleaning. He speaks to her daily on his way to work making her feel like a real person. Both Tom and the mockingbird show their inner beauty through their actions, not their appearance.