Mini Mario Card
From the Super Mario Wiki
Mini Mario Cards, also shortened to Cards, are collectible items with a picture of Mini Mario and a letter on the back of them in Mario vs. Donkey Kong 2: March of the Minis. There are nine cards in each world, and getting all of them unlocks a Minigame.
All the cards together spell out: "MINIMARIO". The level number of a floor corresponds to the letter as it appears in "MINIMARIO" (i.e. Level 2-5 has a card). They reappear in Mario vs. Donkey Kong: Minis March Again! and Mario vs. Donkey Kong: Mini-Land Mayhem!