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!
mrs-campbell's comments are very good. I'd like to add a detail that you might want to address later in your essay, although probably not in your thesis. While watching the jury return to deliver their decision, Scout slips into a sort of half-wake state in which she imagines that the hot summer evening of the end of the trial is like the cold February day on which the mad dog was shot dead in the street. As I read this short and interesting reflection by Scout, the mad dog can be seen to represent the irrational and racist fears of the general population, something that Scout had hoped would be shot down by Atticus' clear argumentation and logic.
I really like mrs-campbell's suggestion for a thesis because the novel seems to me to show that, indeed, logic and reason don't always win. Convincing a bunch of people to change the ways they think is a whole lot more difficult that shooting a slow-moving dog out in the open.
let me give you one...IN TO KILL A MOCKING BIRD ONE OF THE THEMES SHOWN IS THAT YOU SHOULD ALWAYS USE REASON AND INTELLIGENCE TO SOLVE PROBLEMS....A thesis sentence can be as simple as that. FYI this is coming from an 8th grader :)