Maybe this answer exists somewhere online and I’m dumb, but… what benefit does Sudowoodo have using mimicry to appear as a tree? It seems like that is a poor choice for a rock type given it’s weak to water (which intelligent creatures will naturally pour on it).
I suppose my first instinct is to say that, on an evolutionary timescale, creatures who see plants and immediately think to pour water on them are probably a recent enough arrival in the world that they wouldn’t have had much impact on Sudowoodo’s physiology or evolved instinctive behaviour yet. But we don’t really know that’s true; there are Pokémon that modify and curate their environments; there are even Pokémon that tend gardens. The Pokédex says that Sudowoodo looks like a tree to avoid predators, and that does make sense to me; I have no problem with that. So I suppose the best answer is probably that looking like a tree does work really well for its intended purpose – well enough that it’s worth accepting the unfortunate side effect of sometimes having water poured on you.