In some ways, few essay topics are inherently easy or difficult. Whatever your topic, you can choose to work hard or to do more superficial work. That being said, topics that are too broad or too cliched are less likely to result in good grades. So, for example, if you chose to write about racism in To Kill a Mockingbird, it would be very difficult to say something original or interesting. Although more work to begin with, narrower, research-intensive topics are actually easier to write well, because they are very focused and the actual task of writing really just involves organizing information.
For example, there is an entertaining moment when it turns out that one of Scout's teachers, Miss Caroline Fisher, is from Winston County in north Alabama, a county that was pro-abolition during the Civil War. You could write an essay on how the differences between pro-abolitionist Winston and anti-abolitionist Maycomb affect Fisher's role in the novel and her attitudes toward black people.
Another possible essay would focus on the school pageant, where you could do research on school pageants of the period and see how accurately Lee represented them and the role of such pageants in town life of the period.
Racism is a topic that is prevalent in To Kill a Mockingbird. There is an apparent crime against an African American man in the South during a time in United States history when there was no equality amongst races. Atticus Finch, a white lawyer, represents this client without the support of his town. In the book Atticus Finch says that just having the trial, regardless of the outcome, is a huge step in the right direction for equality. He is an anchor to this cause and makes it his point to teach his children and community.
To Kill a Mockingbird is a novel so rich with multiple themes, you should have no difficulties coming up with a topic. The obvious topics are who represents a mockingbird and what changes are seen in the children. However, you also could examine the Romantic values seen throughout the text or even examine how the test itself could be used as a character education textbook. Atticus' life lessons hold true today.