From the Super Mario Wiki, the Mario encyclopedia
Poison Breath is Piranha Plant's side special move in Super Smash Bros. Ultimate. When used Piranha Plant cosmetically turns yellow with red polka-dots, purple teeth, and green lips, resembling a Putrid Piranha. Poison Breath automatically starts charging through one button press, only stopping if it is fully charged, or if Piranha Plant is hit. Once fully charged, purple clouds of poison come out of Piranha Plant's pot or Warp Pipe. Piranha Plant blows a purple, poisonous cloud in front of itself that slowly moves forward before dissipating. Poison Breath does not inflict any hitstun or knockback on opponents, instead dealing damage: in particular, Poison Breath's uncharged variant deals 30% if all of its hits connect, while the fully charged variant can deal 50%+. The poison cloud is also fully opaque, obscuring everything within it.