Hey I noticed a Piranha Plant discussion many pages back and I want to offer my suggestion: I doubt it got in simply because it's iconic, or whatever justification we come up with to go "Of course it made it! How could we not have seen this coming?!" I'm certain that if a wiggler or a smiley cloud or a spiny or Chargin Chuck or even a Noki made it, people would be trying to justify it with some other well-written BS.
PP's a joke character sure, but it's not simply that all by itself. There's tons of joke character candidates from Mario alone that would've shocked us and made us scream WTF. I think the main purpose of Plant is to disrupt our speculation. We've gone on for years analyzing relevancy, importance to their particular series, ties with Nintendo, moveset potential and characters per series, but Plant flies in the face of all of that.
We focused on characters who played a big role in the plot of the games they're in, but got an obstacle as a fighter that we treat as scenery in its home series. We focused on relevancy to weed out most, but Plant is so synonymous with Stage Hazards that we barely acknowledged it. We say "X can't be in since their body has no moveset potential" but got a stem with leaves and teeth on a round head. Plant is Sakurai trolling us who spend years speculating and trying to predict his choices. He's telling us that we're wasting our time; he'll add anyone or thing he wants no matter their potential, importance or popularity (usually) even if they're not bona fide characters because he chose to. He knows for the most part that he can add who he wants as long as they're fun to play as, and people will treat that newcomer like a no-brainer and act like they saw it coming. Sakurai enjoys trolling and seeing our reactions but as long as we keep thinking certain characters can't ever get in we'll see more WTF moments.