Rooted in a side mission found in another game, it seems at first blush like a weird, downloadable lark. To be fair, it might be just that for another developer, on another platform. Quirky experiments don't belong in retail games, and that's exactly what Captain Toad is - a series of shovel hits at a very specific quarry, in all sorts of directions.
Or rather, it's a very pure expression of what Nintendo games are: playable elaborations on a strong, central concept. Nintendo games explore their root mechanics thoroughly, finding fun through solid execution and exploration of the idea. Using that as a verb, Captain Toad Nintendos. It Nintendos hard.