Lady Diana was a member of the British aristocracy as the daughter of an Earl, and had more blood ties to past English royalty than does Charles. Kate Middleton, for all the common status of her self-made, wealthy parents, also has some royal bloodlines. Even the loathesome Camilla has some royal ancestry, including an ancestor who was the mistress of a past king, iirc Victoria's son Albert. So they are a little bit inbred, even though their ties to the Saxe-Coburg and Battenburg dynasties (and Prince Phillip's Danish and Greek royal heritage) are often resented.
She was a commoner. That is indisputable no matter how much she was a member of the aristocracy.