If skin is pimply, it has a lot of small raised red spots, especially on the face. Those spots are pimples. We say this most about teenagers, though actually a chin, back, or shoulders can look pimply at any age. It names the look of the skin, not a disease — a doctor would more likely say acne. Said about a person ("a pimply kid") it can sound a little unkind; "pimply skin" is kinder. Informal, everyday English.