My issue with Geno is how he's the least important party member in his own game. Say what you want about Mallow but at least he had a character arc, Geno's closest thing resembling an arc is going to Mario and telling him he needs to keep doing what he's already doing and then he ****s off forever after that. It's why I'm 100% sure that the Forest Maze meme ended up causing a bandwagon in his favour.
Btw in case people see bandwagon and get upset, I don't think bandwagons are inherently good or bad
Thinking back on SMRPG, you are right. Geno kind of does nothing important to the plot. Peach has a whole section to the game focused on stopping her from being married off to Booster (and proving how she can indeed be able to save the kingdom as well). Mallow has the the Nimbus Kingdom arc. And Bowser gets focus throughout the game (showing him with gradually shrinking troops up until he eventually joins you, his gag in Marrymore, how various NPCs react to him in Monstro Town, and finally his homecoming in Bowser's Keep, with Magikoopa that raised him giving him an unlimited coin box). Mario himself might actually steal the show and is probably one of the most memorable silent protagonists in a game ever (he does a sort of Courage the Cowardly Dog emoting to explain things, and its a really funny riff on the silent protagonist arche-type).
As a fan that remembers media back in the day, the memorable person of the team at the time was Bowser being on your side. I think aside from Geno's attack and abilities, he is probably the most forgettable member of Mario's party in that game. He is just kind of there. I think part of the gravitation towards Geno is the result of wanting Super Mario RPG wanting some kind of broader identity and recognition since a lot of original stuff came from that game that was never re-used. The only choices as far as party members went, were either Mallow, whose crying outright causes rain and is weak physically, or Geno who has bullets for attacks. The choice is obvious. Personally, I never thought Geno's design was "cool" though his attacks are interesting (I would say all of the cast has interesting attacks, like Bowser outright chucks Mario at enemies for one of his).
I think the "the whole game revolves around Geno" is really stretching it. All of the party members are aiming to save their world. Its about all of them. However, if I had to say which party member made the strongest impression on me personally, I would say Mario as strange as that sounds. As I said earlier, one of the most charming silent protagonists ever. The only other silent protagonists to make as strong an impression on me are Crono (who like Mario is simiarly emotive, though not to as a great an extent), some of the Links, and Lucas from Mother 3 if he counts as one (since he speaks up until he becomes the main protagonist). Mario added a lot to the humor and charm to the game, and I am probably the only one out there that really thought he lowkey stole the show (even though the whole game is named after him, and you outright control him and only him the whole game outside of battles).
I think bandwagons are mostly bad personally because they eliminate the capacity for an individual to think for themselves, and have more of an individuality. However, that is my opinion personally. In today's world, people are longing for any sort of identity, and bandwagons are a sort of band-aid for that hole that people have in regards to such longing.
Once again though, I will repeat that Geno is the most likely character for DLC I believe (aside from the highly rumored Ryu Hayabusa). He is getting in, its just a matter of when, as opposed to if. I imagine somewhere part of the second wave. I imagine he either opens it or closes it.