Something about her looks....
When she was first on the scene (Princess Diaries), I thought she was stunning. As she got older, I still found her attractive, but much less so.
Took me a long time to figure it out, but I think I have...
She has very large features (eyes and mouth mostly). When she was younger, she had a rounder 'baby fat' face. The large features on that face made her look like a living animated Disney Princess. Maybe that's how she got the Princess Diaries job...
As she's gotten older, her face has gotten much thinner so her eyes and mouth take up more of it, making them seem even larger and giving an unusual look that's no longer like a Disney Princess.