Thought I might chime in...
If you look at the geometry of the two gyroids, you'll notice that the one that's a stage backdrop has a lower poly count (note the elbows, top of the head), a lower res texture for the face and dots, and also has a lot of "baked in" shading, which is missing from the one that Villager is riding on. Also, the one Villager is riding seems to have a little bit of bump mapping that is missing from the stage object (looking at the ridges of the "eye holes," the right edge of the right eye (from our perspective) is slightly lit, whereas the other ridges are darker toward the center of the eye hole, indicating that lighting effects the texture of the face.
Given the fact that the characters' models are MUCH more detailed than other elements in the game, it's reasonable to deduce that the Gyroid he's riding isn't part of the stage.
I personally think it's most likely that it's an attack, since I would guess that an item or assist trophy wouldn't be as well modeled. BUT, I can't say that for sure, since we don't really have other items or assist trophies to look at in order to reference how detailed they are.