The Mushroom Topping is a small (possibly about 2 ft tall or so) Portobello Mushroom with googly eyes, no mouth and thin, tiny black legs, always having a derpy expression.
It's a light brown colored Mushroom, will follow anyone without any apparent reason.
Friends with the other Toppins:
Pineapple (a little pineapple with sunglasses and white shoes)
Cheese (a little cheese block, with no legs)
Sausage (a pepperoni sausage with a big annoyed brow)
And
Tomato (a round tomato with a round nose