It's OK, every popular character has a lot of haters, even K. Rool and Ridley that are now playable and everybody loves them.
Too generic is like too big and too irrelevant, a bad argument.
It was bad before, but after Piranha Plant it's just dumb.
Piranha Plant is not only just a generic enemy, but it has a very fun moveset that was created using the the entire Piranha Plant species as a base.
Sakurai is the creator of the Kirby franchise, Waddle Dees and Bandana Dee, if he thinks Spear and Parasol are not enough he can literally give his character the abilities he wants and it would be OK.
I mean, that was already obvious with characters like Captain Falcon, Fox or even Meta Knight, but yeah, now thanks to Piranha Plant it's even more obvious.