We've already detailed what the Super Mario Cereal amiibo box does in Super Mario Odyssey, but now you can see some footage of it in action. Not too much to see, really!
So it seems like it's pretty much just a "generic amiibo" that only does what any amiibo can do in a game that can take any amiibo, it doesn't do anything specific.
I mean the amiibo is essentially free with the cost of the cereal, so I don't see why anyone would complain. I expect people to horde some boxes tho D: