I had a friend like this. We would be in public, and she'd pick out every girl that I viewed as gorgeous, and start talking about how ugly she was. She did this because she thought she was ugly and felt the need to down 'prettier' girls so that she'd feel better about herself.
Problem is, she was really pretty. However, she'd sit there and go on and on about how fat she was. She weighed 115 lbs.

Around our guy friends, I know she said things so they'd tell her how "hot" she was.

I don't think she did it out of modesty. She really thought she was ugly. She just needed to be told differently so that she could try to convince herself that what others said was true.