The humor in To Kill a Mockingbird comes from Scout’s observations about her life and the society around her. For most of the novel, she is a very young girl, about 6-8 years old. Her perceptions are those of a child, but the narrative voice is that of a very intelligent grown woman, since the narrator is Scout as an adult.
There are many funny passages, but the humor is subtle and easily missed if you’re not really paying attention to what you’re reading. It is derived from the reader’s knowledge that Scout is simply not old enough to understand fully what is going on around her. The following passage is from the beginning of chapter 10. She is discussing her view of her father, Atticus.
Atticus was feeble: he was nearly fifty. When Jem and I asked him why he was so old, he said he got started late, which we felt reflected on his abilities and manliness . . . Our father didn’t do anything. He worked in an office, not in a drugstore. He did not drive a dump-truck for the county, he was not the sheriff, he did not farm, work in a garage or do anything that could possible arouse the admiration of anyone.
This passage is dripping with irony. Atticus is probably one of the most admirable characters we could encounter in contemporary literature, but from Scout’s immature point of view, he “doesn’t do anything.” Before the novel closes, he will do things that nobody else in Maycomb could or would do. The importance of this is that Scout, despite her youth, comes to realize that Atticus is a truly good man. The humor of her initial misperception gives way to the understanding that comes as she grows and develops intellectually.