Cultivation seems to have gotten a kind of weird change, and rocky surfaces can't be cultivated anymore (neither for grass planting surfaces - a few annoying spots even popped up my my early access launch world meadow base). This applies to all biomes, but is especially noticeable in the Mistlands, which has a lot of those.

That said, there are stretches of grassy lands in the Mistlands - if you intend to build a Mistland Farm for the Mushrooms, that's where you can cultivate the ground and grow them.