Just six short lines, and they pack in a whole scene: a body, or several, stretched out on a beach, seen from the outside. The phrase "a folly of hot hips" is the heart of it. "Folly" suggests foolishness, something a bit reckless or indulgent, and "hot hips" reduces people to warm flesh baking in the sun. The word "beached" in the title makes things stranger. It brings to mind something washed up and stranded, not someone relaxing on holiday.
Then comes the twist. The joy here is "thin / as a late supper," meaning slight, meager, an afterthought. A late supper is what you get when the good part of the day has already gone by. So the speaker seems to sense that the pleasure on display is real but small, maybe a little hollow.
The mood is wry and quietly melancholy. The speaker doesn't scold, but there's a dry, observant tone, as if the sunbathing looks more like drifting than celebration.