To Kill a Mockingbird goes beyond the simple message "racism is bad" to attempt a more complex examination of how racism works. All forms of racism are not the same: some are born of hate, some of fear, some of laziness, some of self-righteousness, some of all these combined. What all racisms have in common in this book, however, is a failure of imagination: the inability to see that even someone who looks, and talks, and acts very different from oneself is fundamentally the same as every other human being. The history of race in the novel, as in America, is based on drawing distinctions solely for the sake of discrimination.
The African-American characters in To Kill a Mockingbird appear only to contribute to the development of the white characters, rather than as individuals in their own right.
To Kill a Mockingbird suggests that racism is learned, and therefore can be unlearned.