I've been teaching this novel for 20 years and have never really viewed Scout as a mockingbird. The real mockingbirds of the novel, I believe, are Tom, Atticus, and Boo. However, if you want to view Scout that way, you can focus on how she is often persecuted through no fault of her own - being criticized by her aunt for not being feminine enough, by her brother for being too much of a girl at times, by both her cousin Francis and classmate Cecil Jacobs because her father is a "nigger lover," and also by her cousin for running around with the "stray dog" Dill. She is also persecuted early on (in chs. 2 and 3) when she stands up for Walter in class, explaining why he can't take a quarter from Miss Caroline, only to find herself in trouble with the teacher and getting her hand hit with a ruler. She tries to do the right thing but is punished for it; she then turns around and takes it out on Walter by trying to beat him up. Some of her behaviors can be seen as mockingbird-like, others not so much.