Diversity and intersections of difference are indeed fundamental elements of To Kill a Mockingbird, particularly as they relate to the coming of age of Jem and Scout Finch.
In the early chapters, Scout demonstrates very little awareness of difference. In the classroom, for example, she struggles to understand why she is reprimanded for explaining Walter Cunningham's lack of shoes or lunch. Similarly, she has trouble understanding why Walter's father anonymously leaves goods in exchange for Atticus' services, rather than deliver them in person. Scout sees Walter only as her friend and classmate, not as someone from a different social class whose poverty could be a source of embarrassment or shame.
The most obvious example of diversity and difference throughout the book is race. In addition to the court case at the center of the story, racial difference and discrimination are present in many other aspects. For instance, the children see Calpernia as something of a mother figure whose authority is to be respected. However, in later chapters, particularly when they attend Calpernia's church and sit with her in the court house, Scout and Jem begin to see the various ways in which African-Americans were treated or spoken of differently by white people. This difference in treatment becomes more apparent when Atticus agrees to represent Tom Robinson. Atticus' decision changes the ways in which he is viewed by both black and white members of the community, most notably his sister, who fears how his decision will affect their family's standing in the community.
Finally, Boo Radley is one of the less obvious but no less important examples of difference present in the book. To the children, Boo is a source of mystery and excitement because he is a recluse who has achieved legendary status in their minds. The reader understands fairly early on that Boo's behavior makes him very different from the other characters, which shapes the way that he is perceived by others. To the other townspeople, he is a sympathetic if not tragic figure, whereas the children see his difference as a source of excitement and danger.
Over the course of the book, Scout and Jem develop greater understandings of how differences of race, class, and social position shape a person's identity. Ultimately, these aspects of identity shape the community in which they live, which is a hard lesson that each character grapples with throughout the novel.