She was cowering under that tiara. That's a frightened, deeply anxious look on her face. Why? Why couldn't she RISE TO THE OCCASION?
If Her Majesty was so kind as to lend you a tiara that once belonged to her mother and her sister, a deep, solemn message is implied there. It's a test. "I consider you one of us now. You are dear. I will remember my mother and my sister when you wear this. Wear it well." In other words, RISE TO THE OCCASION. SHOW GRATITUDE AND PRIVILEGE BY WEARING IT WITH CONFIDENCE. As soon as you see the Queen, curtsy in such a way that you are deeply honoured by her generous gesture. Make her proud that she lent it to you, that YOU DESERVE IT. It will form a quiet bond and understanding. These are ancient, medieval sentiments, but they are still practised in the court of Elizabeth II (even more so when the Queen Mum was still alive).
Likewise, when her father-in-law gifted her with that ruby and diamond necklace, Kate ruined it by covering it with her hair and by slouching. This is ingratitude. If you were Charles, would you be motivated to ever give her high jewellery again? No. So it all goes to Camilla. Or Harry's future wife. No more jewellery for Kate; plates from Zara Home will do.
The Queen herself said that Kate lacks polish. I agree with cleo12 -- it's just not in her, even if you sent her to finishing school. And I suspect that the reason she refuses to refine herself is because in her mind it's her tarty, low-class, doormat, undignified, bimbo self that snagged William in the first place -- and to go the other way is to lose William. The trouble is, as William grows into his role and exposes himself to more and more sophisticated, worldly types, he will inevitably become more sophisticated himself and will outgrow the stupid bubble he and Kate have made for each other since Uni. Right now the only thing that is keeping their marriage together is the Middleton family togetherness rah rah rah act that still seems to appeal to William....... until the day comes that it no longer does.