I think some girls who says they are ugly maybe aren't the most attractive in society's standards, and some are. I think it has less to do with actual beauty and more to do with self-perception. And you can perceive yourself as unattractive, whether you are or aren't.
Half n Half. Some people actually question their attractiveness, and want honest input from others. The others are just seeking attention and confirmation on how beautiful they look. I'd say these type of people do not show up as often as the former description. More like 80% actual questions and 20% self-obsessed people.
I believe some of them really have no idea how attractive they are, while the others are just fishing for compliments but pretend to be insecure. I have noticed there were so many 'rate me' or 'am I fat' questions lately. I mean some of them are incredibly attractive so I don't see the point of asking that question. It's so annoying, that's why I didn't answer them.