Yeh, I think they are sneaking her class status upwards. She said or did something in The Sontaran Experiment that made me feel I had completely misjudged her class. (I can't now remember what it was.) Her grandad comes across as firmly working class (Bernard Cribbens always plays that sort of character) and her mum comes across as working class. Donna herself seems either working class or lower middle class. I think she has seemed more lower-middle around Martha because of the deliberate thing of her raising her game in response to Martha. The next stage will be making her more and more intelligent - bet you anything you like she won't stay as just having common sense for long.