I actually believe her when she says she is happy she didn't become queen. To begin with, she lives in the most democratic and one of the richest countries in the world. Her standard of living is superb. She has a great job and lives an affluent life. She has a family, and her husband adores her, at least judging from photographs he has taken of her showing all his love. Her family is not corrupt, as is the case with several members of the Spanish royal family. She is free to do as she pleases, work, speak, move. She has no need for security. Nobody wants to kill her. There are no scandals surrounding her. Overall, she has a wonderful life.
As queen, she would have zero power over anything, least of all her life. She would have been scrutinized continuously and would only cut ribbons, wear dresses and tiaras, and basically have no interesting things to do. Even her kids wouldn't live a normal life. Yes, she may have been richer than she is (and she is rich), but no rich person actually has access to their entire fortune. No woman who marries a billionaire has access to his billions to spend. There are limits.
Being a pretty object on a shelf is not that desirable for most people. Being happy, successful, self-made, and affluent on the other hand is much much better.
So yes, I believe her.
What a choice of photograph! She kind of looks like Mrs. Trump here.
Haha! ... That makes what sounds like a "sour grapes" statement (being happy she did not become Queen) seem even more fitting!