I've never thought of the word "commoner" as rude. Then again I am in the USA, and we don't have royalty here. Perhaps, its the nicest way to say the person is not born into royalty or the aristocracy in English or British terms.
I am so glad that Diana raised her kids to learn about life outside the palace walls in so many ways. I am glad the boys grew up & became good men who will continue their mother's legacy of giving & being involved with the public.
These days, there are not many royal families left, so it makes more sense for them to either marry someone who is from a rich family or in William's case with his new wife - someone whose family started in the middle class.