The originality of Diana is that she touched people from far wider horizons than other royals (the Queen included). At her funerals you could see a complete spectrum of the British society from punks to city type men, all races, all social classes. I'm not totally sure this is the case for other royals. But clearly, the Windsors have not been short of shinning stars.
Fat Mary was call the people's Princess at her time and was hugely popular.
Side note, a lot of people keep saying that Diana was popular just because she was pretty, but if you compare the kind of popularity Fat Mary (the well named) did enjoyed, compared to that of the stunning Alexandra (Pss of Wales) did enjoyed, I guess you can see that beauty is not always equivalent to popularity. Both fat Mary and Diana had this very 'common' touch, which is what made them so hugely popular IMO.