Mario always leads, but the story has nothing to do with Mario. If Geno did not play a "costarring" role in the game, why would it go all the way through the rest of the game in order for him to complete his mission?
I suppose their is the argument that Mallow and Bowser both accomplish their goals toward the end of the game, but regardless, the end of the game is based on Geno's story. There was also the fact you went through several worlds just for the sake of his story that could have otherwise been skipped.
If having the most important story in a game does not mean you have an important role, practically costarring (I could have said starring but Mario did lead, like you said) I don't know what is.
Anyway, I am supporting Geno far more than I should... I don't want to see him in either.
Remind me, in what Nintendo game was Cloud a main character in?