Well if you wan't to be really specific it was Jumpman, not MarioBut Donkey Kong is related to the Mario games, though. DO you know what Nintendo's first game ever was? That's right. Donkey Kong. Do you know who the main character was? BINGO! It was Mario.
As I said before, I am not opposed to character from unrelated franchises being in Mario games, but the argument you have presented here is kind of moot.