A rubicund person has a red face, and the usual picture is someone plump and well-fed — an old gentleman after dinner, a farmer who works outdoors, or a person whose cheeks are pink from drink. Learner dictionaries often add that the person is fat; that is the stock picture, round and rosy, though the core of the word is the redness of the face. It is a literary, slightly old-fashioned adjective. You would find it in a novel, not in a doctor's notes (a doctor would say flushed or ruddy). It describes a look of health or indulgence, not a rash, a sunburn, or a face red with anger. It is not unkind by itself, though it can be used with a little smile.