Everybody has ab muscles that would look great if they had no fat over them (subcutaneous fat: the fat just under the skin and over the muscle), so reducing body fat will let them show. However, the thicker and stronger your ab muscles are the more they show at a given body fat level, so developing your ab muscles more will help as well (and the exercises you do to develop your abs will help reduce body fat, although you can't out exercise a bad diet.