I am sorry she's going through that. Sadly, 9 is about the age that girls begin feeling the pressures that are placed on them by society. Sometimes it's because of what they see. Sometimes it's because of what others see and say.
In 2010 (maybe 11), our then 9yo daughter told us she didn't smile anymore because of her "buck tooth." We had never used that phrase, but someone in her class did as they laughed and pointed. She had always been very confident, singing solos in school shows (which may not sound like much, but this was in front of several 100 people), cast as the lead dancer, leading her Girl Scout troop in projects. The person who said it was likely jealous of her being asked to demonstrate a song in music. It seems like such a simple time when we remember it, but kids can be awful.
The point here is that kids, particularly girls, are going to feel different and ugly for various reasons as they begin to think about the world around them. Masks are a symptom, not the reason.
I hope your niece is feeling better about herself. It really is a long journey, but our now 20yo is back to her confident self after some tough years of fighting her natural personality and self in an effort to fit in with others. She's a joy and has told us that she's grateful for our support and love when she struggles.
That's probably way more than you wanted to hear on this board, but it's meant as encouragement.
in general what girls have to deal with through their formative years and beyond terrifies me. 