When a fighter picks up the Ramblin' Evil Mushroom, a cloud of spores will shoot out by pressing the attack button, putting a small mushroom on the opponents's head. This will cause the horizontal directional inputs on the control stick to be temporarily reversed. Hitting the opponent with the spores three times will make the mushroom grow larger and exude spores, extending the duration of the effect. Attacking the Ramblin' Evil Mushroom without picking it up will cause it to shoot out a cloud of spores, putting a mushroom on the attackers head. However, the attacker's controls are not reversed and the mushroom cannot grow larger.
This effect references the Mushroomization status the Ramblin' Evil Mushrooms can inflict on the player in EarthBound, which causes the overworld controls to be rotated. This effect is shown by a mushroom appearing in the party members' heads, much like the Mushroom growing on opponents in Ultimate.