I was surprised to notice that pawns were still complaining about sleeping on the ground despite using ancient beds. It turns out the condition for applying the mood penalty is as follows:
if (bed == null || bed.CostListAdjusted().Count == 0) actor.needs.mood.thoughts.memories.TryGainMemory(ThoughtDefOf.SleptOnGround);
Ancient beds satisfy this because they have no cost list (material cost) since they can't be constructed. The logic feels like it was written with sleeping spots in mind, so it could be considered a bug that ancient beds are treated the same way.