It was an error , they changed it.
PP is free for a reason , she was just a character , no music and stage. She's supposed to be that silly bonus character.(and because that make the roster count a sweet 70 if we don't count echos. Ending on the 69th character beeing a buffed Fire furry cat wasn't a good move , even if It's not that important.)
And I know that PP being in a pot seem to be a nod to the SMRPG commercial , but potted PP already exist in Mario games.
And besides , except the underground theme as a new remix (again) wich music could she bring with her ? Since PPs are often see in 2D/3D platformers , and they have most of their music covered , wich music could they use ? I mean , even 3D World got a music track...
And we already have a lot of Mario stages , especially classic ones , I think they won't be able to make another one that stand out.
(And yeah I'm calling PP a she , even tho plant don't have genders.)