I think she is trying to expose the racism that exists in the world. People who have lower blood statuses may represent the victims of racism, social classes, or possibly the minorities.... Many people discriminate against others based on social class/the amount of money they have.
It's saying how Purebloods and rich people have more rank than others, even in the wizarding world where muggle borns may be more powerful than the purest wizard. Take Hermione and Neville. Neville came from an old, respected family. Hermione is, of course, Muggle born.
Its showing rasicm in the world today and from the past. If your a pure-blood yuo rich welll known and believed to be a dominate force in your eyes as if your better than everyone else. Half-bloods are sort of the middle class the people that only haf of a true wizard and are respected because there are so many but seemed weaker becasue of there one muggle parent. The lowest class were muggle-borns which purebloods believed shouldt exsist becasue it wasn't natural and shouldn't xsist becasue they have booth muggle parents and have no magic in them what soever but then again it comes back to the perspectives that purebloods have that dominate the idea of rasicm in the wizarding world since there seen so high and mighty there voices are heard and there listened to more than mugglebornss or even half-bloods.
I believe J.K Rowling is trying to show how even though the Wizarding World has all it's wonderful little quirks and aspects, there's still a darker element to it where those with wealth and a "pure" blood heritage ultimately hold power over those who aren't purebloods or very rich. The Wizarding World isn't perfect and has it's own problems and prejudices.