Hell! I would have rather prefered a Goomba or A Koopa Troopa (At least those those guys can move around, punch, kick, and fly) as opposed to a plant that has nothing interesting to offer us!!!!!
As a Goomba supporter (heck i made the support thread), I'd argue that Piranha Plant has MORE to offer than goomba or koopa!
Before I start, I just want to be clear that with 75+ characters in this game, the chances that new characters will have something even slightly similar to another is hugely likely no matter what. Simon's Axe is a projectile that goes in an arc like Mii Brawler, K Rool has Dedede's Nair, a counter, and the crown is a boomerang like Link. Incineroar has K Rool's drop kick, Pikachu's nair multihits like Mewtwo's, etc. Moves are gonna be shared sometimes, it happens.
With that out of the way, I'll explain why I think Piranha Plant offers more than you think to the roster.
-Neutral Special: Weird spiky ball thing. This is entirely a new move, blowing a spiky ball in the air which covers opponents landing, and can be shot away to stop recoveries from off stage. Has lag, but a weird and different move nonetheless!
-Side Special: Toxic Gas. This is nothing like Wario's fart which does damage and charges and kills. This is a toxic gas that moves slightly forward and is implied to do damage over time, which could make this a good way to rack up damage. I see potential set ups of setting the gas at high percent, grabbing and pummeling and having the gas do damage at the same time.
-Up Special: Helicopter. This is different enough from K Rool's, his is not meant to be used for damage, but from the appearance of Piranha plant's this is a multihit move with even more maneuverability than K Rool's, and has a much larger hit box.
-Down Special: Expand Plant. This is entirely new and is HUGE on range. This move is super cool, and you can angle where you can attack and covers half the stage. This could definitely be good on stages like battlefield with platforms attacking from one to another, and I think its just a really weird and cool new mechanic we've not seen in a character before.
-Final Smash: Petey Piranha. An amazing callback to the first boss of subspace emissary with the cages, it makes total sense and looks really cool to see petey help out in the fight!
Thats for his specials. As for everything else, i could go on for a while on why each move is different but thatd take too long. Sure nair is like Ivysaur's, and Dair is like Dr. Marios which is also like Ganondorfs which is also like Captain Falcons. But hey, dunks are always hype and youre lying to yourself if you dont think so. His fair is solid, his forward smash looks cool, and overall hes got a solid looking moveset.
Finally, the character sparks tons more personality than goomba and koopa could ever show. His toothy grin, his silly tounge hanging from his mouth as he runs, he looks like hes enjoying every second of getting to slap around the competition! It's like he's so psyched to finally stretch his tiny feets for the first time and take the fight to the opponents than waiting for mario to accidentally step on him. He's goofy and he knows it and relishes in it.
I hope i could help you understand a bit more about this character and why i love him. You don't have to love him, but i just want to help show you why I personally dont see him as a copycat of other characters in the game, and shows off alot of personality for a common mario enemy!