Many titles could be appropriate for such an essay. Before you begin writing, you should pick specific examples of symbolism in the novel and talk about those. The largest example of symbolism in this particular novel is the use of birds (specifically, but not only, mockingbirds) to represent innocent people that are harmed by racism in Maycomb. Tom Robinson is referred to near the end as a "songbird" as he is a victim of racism, and Jem and Scout's last name (Finch) also indicates that they might be victims of racism.
Once you've decided what to write about, you can pick a more specific title if you so choose. A generic title like "Symbolism in the Novel, To Kill a Mockingbird" would work just fine, but you could get creative and go with "The Symbolism of the Mockingbird" or something similar.