Most guys I know prefer tits and ass, I've never seen a guy who preferred great abs. Though it is a bit creepy. I'm sorry but being 90lbs at age 16 is utterly ridiculous, same thing with models. What is so wrong with a girl with curves?
well sticking to the original question, yeah it is normal for a gir who is 16 to have abs like that. it is perfectly normal, and the more you exercise, the mroe weight you lose. breasts are made out of 90 some odd percent of pure body fat, so probably the more she worked out, the mroe her titty fat went away.
as for my personal opinion, i htink girls who are fit like that is sexy. i dont really look for figure in a women, im more about the inner workings sincei m looking for a serious relaitonship.