A Mockingbird to my understanding is somebody who doesn’t cause trouble to anyone has has an essence of innocnece to them as well as mockingbirds only make music for everyone to hear and doesn’t hurt anyone. One such example of a mockingbird is Boo Radley.
One perspective of how Boo Radley is a mockingbird is the simple fact that he never commits a selfish act or deed. He leaves everyone alone, this might be due to the fact that he is despised for one reason or another. The other idea is his back stories and his interaction with Jem and Scout as he is kind to Jem and Scout because they are treating him like a noraml human being despite his uncertain past.
React!