The main themes in this novel are both friendship and ancestry. In regards to friendship, Raheen and Karim grow up as good friends. In fact, they treat each other more like twins than simple children whose parents are best friends. When Raheen goes away to college, she experiences some resentment from Karim and doesn't understand why. Raheen eventually learns that her own father stopped courting Karim's mother long ago only because she was a Bengali. This is enough to repel Karim, but only for a time. Raheen reacts to her father's issues, eventually forgives her father. The story ends happily with Raheen and Karim together.
This leads us perfectly into the theme of conflicting ancestries. As you can see, it is ancestry that almost separates Raheen and Karim. Karim reacts violently in learning the real reason why their two parent sets are only "best friends" and why Karim's mother isn't actually married to Raheen's father. (No wonder the two felt like twins!) The answer is ancestry and class. It wasn't acceptable for Raheen's father to marry a Bengali woman; therefore, Karim's mother was out of the running.
A diaspora ... is a scattered population with a common origin in a smaller geographic area.
In this regard, this could certainly be considered a piece of diaspora writing! Note the theme of separation with ancestry and class! There is a common origin here with Karim's mother and Raheen's father, but a reaction that scatters them because of Karim's mother's Bengali heritage.