To be fair , I think that Yaya has always be naughty and has always like to act funny. I think she was shy at first because her Thai wasn't good , she was very young and everything was new to her.
Now , the language is not so much a problem and her confidence has grown so she is able to be more herself actually. Nadech has helped her along the way, I am sure , but I don't think he has changed her in anyway.
Yaya herself said something about not wanting to be a princess because she won't be able to be naughty lol ! The funny faces were already there when you look at early pictures of her and she says in interview that she does have tomboyish taste .
But all of this does not stop her to have good manners and show respect and kindness to everyone. The same goes for Nadech. You should not worry really ...