Yes, her comment was rude regardless of her size, but given her size, it was even more hurtful. The OP was venting and I don't think she TRULY meant literally that all skinny girls are evil.
I used to be really thin, too (108 at my lightest) but I wasn't a rude b***h. I was raised better.
Making fun of, and discriminating against, fat people is one of the last "acceptable" prejudices. As current and former fatties, we are hypersensitive to that and likely always will be. A good thing. Having a degrading comment about the size of your behind hurts coming from anyone, but it has an extra sting coming from a thin person.