I have to come up with a thought-provoking thesis statement for my To Kill a Mockingbird essay, and I've written one which I quite like, but need a...

This thesis statement is rather good, but it could use revision so that each of the three opinions are of parallel structure.

Try this, then:

Despite his seclusion, Boo Radley plays a key role in the lives of the Finch children because of the mystery surrounding his existence, his courageous act of protecting the children, and his inspiration to Scout in her attainment of a moral conscience. (Mystery, act, and inspiration—the three points—are all nouns, which creates parallel structure.)

Also, "Maycomb's society" is changed to the lives of the Finch children because there are probably people in Maycomb who do not know about Boo Radley (Scout alludes to "neighborhood legend" rather than town rumors). And, since the novel is a narrative of the maturation of Jem and Scout, especially, the focus of the thesis would be better made if you were to limit it to the lives of the children rather than including the entire town.