What is a good thesis statement about To Kill a Mockingbird with the theme of using reason and intelligence to solve problems?

This theme for a thesis statement is a good one, because that is exactly what Atticus does in the trial--he uses logic and reason to figure out the situation that occurred on that fateful day that Tom was arrested.  Logic, reason and intelligence, all clearly shown by Atticus, pointed out that Tom was innocent.  Unfortunately, despite the reason and intelligence that proved Tom not guilty, the jury let their fears, prejudices, and racism (which is centered in fear and a total lack of reason) get in the way of thinking clearly and doing the right thing.

A thesis statement is usually one sentence long, and gets to the main point of the matter that you would like to focus on in your essay.  So, for example, it could state, "In To Kill a Mockingbird, Harper Lee shows how fear and racism is often more powerful than reason and intelligence."  Then, using the example I stated above in your body paragraphs, plus others from the novel, you can show different incidents where fear and racism ending up being the driving force behind people's actions, not reason and intelligence.  Another good example to use is the angry mob that goes to the jail to do harm, and how Scout helps them to see reason through her humanizing impact.  Using such examples would support the thesis statement above.  I hope that those thoughts helped; good luck!