It doesn't help, but contrary to popular belief, it doesn't hurt either.
The last time we got a DLC announcement prior to a game's release, was for the Wii U. The 3DS version was already out for some time, along with it's Mewtwo trophy. So it's likely Mewtwo as DLC wasn't considered until near the end of development. We don't have that prior release muddying things up for the Plant, though; he was far enough done to have a gameplay trailer in his announcement before release, whereas Mewtwo only had a model until much later. For all intents and purposes, Piranha Plant was treated like a base roster character.