I have pretty much always been skinny except when I was on high dose prednisone. I got up to 142 (I'm 5'7"). While 142 is probably considered normal for my height, I looked terrible. I don't carry weight very well. It isn't spread through out my whole body, it just goes to selected areas and I look like I weigh a lot more than I did. Especially with the "moon face" from steroids.
I have weighed as little as 100 lbs when I was at my sickest and I looked so unhealthy. You could see every rib and my head looked too big or my body. I had the body of a boy. I like my weight to be about 120-125, currently I'm stuck at 128. The only reason I am currently trying to lose an extra 5 lbs is because I have a shit load of jeans and I refuse to get rid of them and buy new ones. If I lost an extra 5 pounds they would fit me like they use to.
My cousin on the other hand would look sick at the same weight I am (she is the same height as me). I think it depends how your body is built and how you carry weight.