I think that overall, it is a positive commentary on human nature. There are a lot of negative portrayals of people, but they are balanced with positive ones. On balance, I think the positive wins. Although one of the mockingbirds dies, the other rescues the children.
For each negative event, there is a positive one. Although Boo is portrayed as crazy and scary at the beginning of the book, he also sews Jim’s pants and puts a blanket on Scout’s shoulder.
Even though racism is a dominant theme in the book, there are positive examples. Dolphus Raymond is an example of one of the townspeople who is not racist. Heck Tate is also more open minded.
For each Bob Ewell in the book, there is an Atticus. The book actually portrays a realistic world, expect that the good wins in the end- when Boo Radley kills Ewell.