Marie-Mathilde I think you have brought up a very interesting topic pertaining to why Cookie was regarded so favourably in her youth. She was rather lovely in a wholesome way, but I do not think she was the ravishing beauty that many said she was. Mary Harewood was undeniably striking. But what I think drew people to rave about Cookie was the fact that she was the first member of the family to be seen smiling so brightly and consistently in public. It is noticeable that Cookie really 'smiled', bearing her teeth, and I think she enchanted everyone. I think people came away from seeing her rather dazzled not so much by her looks, but probably more so by that smile. It was a natural part of her persona and unlike others in the RF who smiled 'politely', Cookie's was irresistibly wide and bright and she seems to have smiled with her famously blue eyes too, from the inside out so to speak!
Interestingly, the present Queen has that smile too. It seems to transform her when she does it!