The whole 'mudblood' thing is presented in such a way that it is obviously warning readers AWAY from racism and prejudice in general, in that it's only the unsympathetic characters (Voldemort, Malfoy) who are derogatory about Muggles. And Hermione's parents are Muggles. But I do think it's a good way of introducing kids to the idea that other people have views they may find abhorrent, and giving them ways to deal with it, and even setting the example that it should be shouted about.