Scout's humor comes mostly from her attempts to understand adult ideas that are too complex for her. For example, when Scout talks to Mr. Cunningham in front of the angry mob. When she shows up, the innocence of her question diffuses the situation and reminds everyone that what they are doing is wrong.
Scout is also funny because she is a tomboy and therefore acts unusual for a girl. She gets upset when she is accused of acting like a girl! Scout is constantly trying to fit in with the boys.
Finally, Scout is humorous because she is precocious and when we see a little girl acting like an adult we often laugh. The ladies laugh at the things she says during the socials. Scout is embarrassed by this, and notes that Miss Maudie only laughs at children when they are trying to be funny.