You know, call me crazy, but I think the choice of
actually makes some amount of sense.
But not how you think.
Here's my theory, Piranha Plant wasn't chosen
specifically to be a WTF character (although it
absolutely serves that purpose); it might have been chosen because some people were asking
(in the ballot) to play as
Petey Piranha (Boss Pakkun in the JP version), which is obviously unfeasible (the meme argument against
applies
100% to Petey Piranha; but
unlike Ridley; Petey Piranha is just a bigger form of a manageably sized character).
I thought of this while watching online reactions, someone (I think it was AntDude) referred to Piranha Plant mistakenly as "Petey Piranha" (
before the Petey Piranha Final Smash was shown) which is what jogged this thought.