Literally the first google result: "The definition of the main character is the character who the story is mostly about or the point of view of the story." Said same website also talks about the difference between protag and main character.
Now it's funny, by that definition, it explains both Geno and Mario in that order: The story is mostly about Geno, but the POV is mostly from Mario since you "play" as him the most (since you literally move him around on the maps and stuff), which is literally what I said in that both Geno and Mario are the main two characters.
TBH to me, it's even funnier that people claim a "main character has to have character development", but man, there's so many characters that don't develop at all that people just absolutely love and adore, and it'd be impossible to call other characters the "main" character of their game. I'm not saying it's good or bad writing; I'm not here to judge that, I'm just saying that because a character gets character development doesn't mean they're more or less main.
Not to go too OT, but let's take a look at Undertale: A game that has virtually zero development for the playable Human character, as you never speak, never emote, never even have a different sprite as you do various deeds or misdeeds (and you can do some horrible and unspeakble misdeeds in this game) throughout your entire adventure. The POV is very obviously from their eyes since you're playing the game the entire time as them. Meanwhile, a lot of characters that aren't you get actual development as the story and timeline(s) go on. The playable Human character is essentially Mario in SMRPG, in that you play as them and see the story from their POV, but still get virtually no character development from the game. At the same time though, the entire story affects the Human (characters interact with the Human specifically) and is affected by them (they will react very differently based on your actions and who you choose to spare or kill). This is the same as Geno being affected (Star Road destroyed) and also affects (redirects Mario to the goal of collecting star pieces and defeating Smithy) the story in SMRPG.
After all that, would you still say the Human in Undertale isn't the "main character"? If so, then you're also saying neither Geno nor Mario are the main characters in SMRPG, but Monster Kid, a character that has no bearing on the story (but is affected by your actions), is more of a main character just because they have some character development at some point Telling Undyne, their idol, to back off of you in Pacifist, or telling you to stop killing monsters in Genocide.
