I’m pretty sure lightsabers still require some force (in the Newtonian sense, not the Force™) behind them, they don’t just melt through everything with no resistance. Think of the scene from Phantom Menace where Qui-Gonn has to cut through the blast doors on the Trade Federation ship. He was clearly putting a considerable amount of effort into cutting through the doors. I’d imagine a thin floor could be cut through, but the lightsaber would have to be dropped at the right angle, and very forcefully. I doubt it could get through a thicker floor, or even a second thin floor.