Wouldn't it be both? Sometimes people genuinely just want an outside perspective on their physical appearance. Other times, they may be very self conscious and so, knowing how a lot of people online, GaG included, sugar-coat their opinions of the appearance of others and can easily rate someone who's more like a 5 with a 9.
Honestly the whole rate me area is a bit of a minefield of psychological reasons - I find it easier just to give benefit of the doubt in the rate me questions - I am always afraid of calling someone out as an attention whore and it turns out they genuinely have low self esteem. The thirsty gagger syndrome doesn't make sense to me because does anyone actually think they will get anywhere by blanket bombing girls with attention until one cracks.
I think it is some of both. So far I have never seen anyone look absolutely ugly, although most posters seem as though they have low self esteem the majority of them look fairly decent, or even good looking... which leads me to think that most of the times it is fishing for compliments. It could be genuine curiosity but I think the latter.