Coming-of-Age; Southern Gothic
To Kill a Mockingbird isn't just Scout's coming-of-age story; it's also Jem's and Dill's. But mostly we hear about Scout. Over the course of the novel she learns to act in a more adult way, even a more ladylike way, and to see the people around her as actual human beings. And the novel is also about growing up more generally, asking an important question: Is it possible to become an adult, to join an adult community, and still keep a child's sensitivity to injustice?
And now for the fun stuff. To Kill a Mockingbird has a haunted house (the Radley Place), a ghost (Boo), inexplicable evil (Bob Ewell), and terrifying nightmare encounters (the midnight raid on the Radley Place, Bob Ewell again). That makes it Gothic. And, like the works of William Faulkner and Flannery O'Connor, it's set in a South still haunted by its racist past.
That, Shmoopers, gives us the Southern Gothic. But don't worry: the book's also got ham costumes and public pantslessness, so the potential horror of the novel is offset by its humor.