What would be a thesis statement about the racism in this novel?

A thesis statement should have a solid argument tied into it. We generally do not write them for people on this forum. Isolating what exactly you mean about racism is important. So, an example might be, "In Harper Lee's To Kill a Mockingbird, racism shapes the character of Scout Finch." Then you would be writing about how racism plays a part in shaping Scout into the girl she becomes by the end of the story.