What is a good thesis statement about social inequality in Harper Lee's To Kill a Mockingbird?

This is a great question, as social inequality is one of the most important themes in Harper Lee's To Kill a Mockingbird. While it's important that you come up with your own thesis (as I'm assuming you're writing a paper on the topic), I'll get you started with some pointers. If I were writing a paper on social inequality in Mockingbird, I would focus on the ways in which poverty and race play into it. 

In the novel, there is plenty of inequality between white characters. Usually, this inequality is centered on income. The Ewells, for instance, are a family with a bad reputation, a history of poverty, and, as a result, a low social status. The Finches, on the other hand, have a history of wealth, land ownership, and a certain social respectability. It's true, of course, that Atticus Finch's family is not rich; however, they are relatively wealthy when compared with the Ewells, who are miserably poor. As such, we can see that social inequality in Maycomb is often based on the history of one's wealth. That said, even the poorest white citizens are regarded as socially superior to the most respectable black citizens. In that case, it's clear that social inequality is created not only through poverty, but also through systematic racism.

All in all, a thesis might be written as follows: Social inequality in Maycomb is reinforced by both poverty and racism, with poor, black residents being considered inferior to all other residents. That said, I would encourage you to think about the topic and create your own thesis statement based on your own analysis. Mockingbird is a tremendous novel, and reading it is a truly important experience.