Hey and thanks so much! That's something that could definitely be made a bit clearer / fleshed in the rulebook - you've just inspired me to make a quick little monster manual, with more move-based statblocks, when I can! I can't promise speed, but I'll definitely reply again to your comment once I've done so, so you get the notification!
In the meantime:
Basic monster types are on pages 45 - 46 of the main rulebook, and combat as a broad category is on pg. 22 - though as you noted, that mainly gives lore, some basic move types, and health. They're categories, for DMs to slot monsters they want to build encounters around into if they already have a sense of what powers or effects they have.
You can actually use that to take monsters set up for D&D 5e and convert them to Mushrooms! Pick a monster you like, figure out what type of beast it most sounds like it'd fit with, and give it health in that range (and some lore / flavor from the category too, if you're feeling it!). You can still use all the moves the monster has in the source material as reference too; just know that "standard" damage in Mushrooms is 3 points, and that strategies for countering a monster's attacks or effects are more based on players having good justification and a high roll, rather than pure mechanics.
Systems based on Powered by the Apocolypse-type rules often don't have monsters take action turns of their own (that's usually reserved for enemy NPCs); instead, monsters only take moves during combat as reactions to players, with the scale of any damage or effects they deal being the flip of the player's roll. If a player only manages to create a small effect, a monster gets a chance to do a big effect (and vice versa).
Hope that helps! & I'll have more info soon(ish)!