Piranha Plant is the first DLC character in Ultimate, the second being Joker.
Piranha Plant is the only DLC character that is free (for a limited time).
Super Smash Bros. Ultimate is actually the first game ever to have Piranha Plant as a playable character (although Petey Piranha is playable in the Mario spinoffs).
Piranha Plant is considered to be one of the most unexpected and controversial inclusions as a Smash fighter: despite being an iconic Mario minion.
The Smash Piranha Plant makes references and shares the traits of multiple Piranha Plant subspecies.
This is one of the few depictions of a normal Piranha Plant having feet. Most depictions of the Piranha Plant are implied to be rooted to the ground, their pipe or their flower pot. The Ptooie, Stalking Piranha Plant and Elasto-Piranha Plant actually do have feet: but it's unconfirmed if they are identical to regular Piranha Plants or if they are subspecies. Ptooies and Stalking Piranha Plants are also not depicted with a flower pot: although the Elasto-Piranha Plant does have a warp pipe that it wears as pants and can still walk with its feet.
Ptooies also puff spike balls into the air in a manner similar to the Smash Piranha Plant.
Piranha Plant changes its coloration into that of a Putrid Piranha when using its poison attacks.
Piranha Plant's extremely elastic neck is possibly a reference to the Stalking Piranha Plant, the Piranha Creeper or the Elasto-Piranha Plant.
Piranha Plant's up special is a reference to the Jumping Piranha Plant: which can also use its leaves to hover.
Each of Piranha Plant's (currently confirmed) 7 alternate skins are a reference to Piranha Plant subspecies. 
The 1985 Piranha Plant skin has the classical green Warp Pipe. Half of these skins have a warp pipe instead of a pot: and these pipes use a completely different model as the pot (most noticeably missing the holes that the pot has).
As Piranha Plant's trailer showed; an 8-bit Piranha Plant is also a stage hazard in Mushroom Kingdom.