The symbol that shines through is the symbol of the mockingbird in this chapter. Remember in chapter 10 when the kids were told not to shoot mockingbirds because all they do is make beautiful music for us? The mockingbird doesn't do anything to harm us. There are other birds that do create problems, but not the mockingbird. Thus, we shouldn't shoot it and brutally murder it.
Here, Tom is being treated like the mockingbird. He is innocent of the crime he is being accused of, and his name is being smeared in the process. Mayella and Mr. Gilmer both tried to paint him as a terrible man, but the truth is Tom Robinson helped Mayella for no charge whenever he could because he was a gentleman. This is the innocence of the mockingbird. He "made beautiful music" for Mayella and did not harm her.
I have heard people looking for more symbolism in the chiffarobe, but I don't really see it, maybe someone else with expertise could refer to that.