This is an in-joke for foreigners in China, and Chinese too. Yes. Sort-of. If someone buys and furnishes a house they'll likely have a proper mattress bought from Slumberland, IKEA, wherever.

But a lot of rental places, especially the older, cheaper, and free ones (a lot of schools have free accommodation for their foreign teachers or students, dorms or a dusty beat-up apartment) and there are some several hundred thousand of these 'foreign friends' in China, so several hundred thousand in the same boat, have a few sheets of plywood with a few sheets run over them. That's the mattress. The landlords don't care, they don't need to, and in their eyes it was fine when they were growing up, so why shouldn't it be fine for you?

And there's the local thing, whatever the bed, nice or not, of putting a bamboo sheet over the top and sleeping on that when the weather's hot. Which I'm not a fan of. It's not getting caught up in the wrong thing, not at all. Come over to s/ccj if you're interested in any more of the 'differences' of life here - it's a satirical sub BTW that was banned from Reddit, banned twice.