First, i'd say don't worry about your image so much but look to your fitness. If you're healthy at 96 lbs then be happy with that. As for squats to remove potential build-up at your butt, many will say and have said many times before: You cannot spot reduce. Your genetic structure will determine where fat goes and where it'll leave first. If you want to lose fat, that requires cardio, if you're gaining weight by increasing fat, then you're going to get fat built up. The best way to gain weight is through muscle (plus muscle weighs more than fat). But again, if you're healthy, there's no reason to gain weight, if you're genetic structure says you should have a skinny frame, you're stuck with it! Unless you want to build up fat or you take the time to build lean muscle mass. So I'd suggest sticking to those squats, and if you're scared of building that fat, then do some running, burn the fat off, plus it'll build the musscles in your legs. If you're so skinny, the fat shouldn't be too hard to remove.