False equivalence. Geno isn't even THE main protagonist in his one game unlike all the other characters you've just mentioned. He's a popular second-string party member in a Mario RPG.
That's an even worse position than being a one-shot Zelda character, since at least one-shot Zelda characters are treated as relevant to their franchise by Nintendo. But being a one-shot character in a spinoff for Mario is not a recipe for relevance.
Is there anything resembling a Geno fansite anywhere? I'd love to see it.
It's too bad the best Geno's ever gotten is that one really brief cameo in Superstar Saga (which they unfortunately had to reveal that Square held too many rights to him for comfort):
But you know what, I do support ALL of the Mario RPG characters getting more recognition. Maybe not necessarily as playable characters in Smash, but maybe Assist Trophies as well as appearing as collectible Trophies.
And I'm tired of having the RPG characters being exiled from the Mario spinoffs. I'm sure everyone will agree that they'd prefer Geno, Goombella, Mallow, Fawful, Kammy, and everyone else over multiple forms of Mario and Peach.
I even found a fantastic poster from a dedicated fan, too!