Well, Geno is recent, but even if he is not relevant (yes, I know for a fact that his recent appearance is a remake based on an old game), while Geno may not have been the lead character, technically, the game he hails from is still a classic masterpiece, one of the greats, and despite that game's being only in two countries in terms of release and being released at the end of the SNES' lifetime, it was still pretty loved, and the remake can help spread that love. More people will get into it, and it will be in the middle of the lifetime of a successful system, the Switch, with the help of the internet and all. Even if the game itself was irrelevant, it was, and still is still pretty loved, and technically to me, that backs Geno in some way. Maybe not completely, but think about it, if the only game Geno was in, Super Mario RPG, was not popular, good, successful enough, etc, Geno would have less of a chance of getting in Smash.
And Sakurai would have less of a reason to want him playable if Super Mario RPG was not a good game in any way, especially in game quality.
Come to think of it, in terms of Super Mario RPG helping out Geno, I think that even with possible irrelevancy, the game's success and more so, the game's quality and everything behind it could have an effect on Geno's chances altogether, if to an extent. It won't be just Geno the character that helps him out, but to me, the game, and the remake, if it is good, has a good chance of helping him out, making his chances better for him in some way.
You can't just have a good character by themselves to be a candidate in Smash, but everything around them from the rest of the game, too. Even if Geno was not the most important character (and he did play an important enough role in the story), the game he was in is big enough to give him some star power, and add to him more than just his character and role alone.