To be fair I get the Piranha Plant disdain, that would be a character people would complain about even if it got in Mario Kart.
Like, it works, and it's DLC so you need not indulge if you don't like it... but I'm not gonna pretend there weren't choices with merits more prevalent out there. Usually when Sakurai chooses a surprise character, it may be unexpected but its qualifications are self-evident. WFT, ICs, DHD, Pokemon Trainer, even Corrin and ROB.
The most that can be said about the Piranha Plant (that wouldn't be subjective) is it has a moveset that works (not a particularly unique accomplishment) and that it's an easily identifiable character. And people will enjoy it, but I'd argue even more people would enjoy a different choice.
Now that it's included the opinion will change, but virtually nobody would be defending it prior to today. This isn't a complaint (given that I can pass up on the character should I choose), more just a concession towards the lack of affinity.