Nintendo may have not created the art assets in this game but this IS Nintendo's Mario. Anyone looking at it (and the scenery) would ID it as Suler Mario Bros 1. It's derivative out the metaphorical ying-yang. The character and the environment (as I've seen in screenshots).
Mario, in and of himself, is a work not simply a concept. He is way to defined for "concept". A plumber who saves a princess is a concept. A plumber with a big nose, red jumpsuit, red hat with some emblem on it jumping on monsters acorns call goombas is very defined. And video games are a medium recognized by copyright law. Saying it must exist physically is very broad in copyright, mostly just requiring it be recorded somehow. Not just in your head.
That not even including the trademark issues.