Although "skirt" can refer to the bottom part of a dress, I think we should reserve the tag itself for the garment that hangs from the waist downwards. When we think of "skirt", we traditionally think of the latter, not dresses. Thus, when we search for skirt, we're going to expect the waist-down garment, not dresses. Implying dress to skirt would therefore muddy the waters without providing much benefit.