What is a good thesis using morality as a theme in To Kill a Mockingbird?

The theme of morality is a good one in To Kill a Mockingbird.

The first step in creating a good thesis statement is to narrow your thesis down to one point that is debatable. 

If you are writing on the theme of morality, you might want to write on a topic such as courage.  If so, here is a possible thesis: "In To Kill a Mockingbird courage is the most important virtue and what separates good people from everyone else." 

Not all people will agree with this statement. This is what makes it a thesis. That said, this thesis is also defendable. This is what make the thesis a strong one. If you wanted to support this thesis, you can mention Atticus's courage in defending Tom Robinson in the face of a mob. You can also include his conversation with Scout when he says that they will lose. Here is the quote:

“Atticus, are we going to win it?" “No, honey." “Then why—" “Simply because we were licked a hundred years before we started is no reason for us not to try to win,” Atticus said.

These thoughts should get you started.