At least there is precedent for Daisy being a main series character, even though she is a one-shot in that department. Geno has to be not a main series character, not a recurring character, and not a franchise main character all at once. Waluigi at least has being a spinoff staple, even though I think that he's also a pure spinoff character should wall him out.
Hardly. Daisy wasn't this tremendously requested character during any Smash, and Waluigi being purely a spinoff character represents a whole different side of the Mario realm. I think it's safe to say Waluigi comes to mind before Daisy when talking about Mario spinoffs.
As for your impartiality concerning Geno, I'll give you a basic outline of why I want him. First and foremost, Geno's popularity is commendable. Practically no one-off characters who aren't the face of their own game (and in most cases, I'd argue even then) receive this much fanservice. Very clearly he's had an impact on a lot of people who've played his game. Of course, you're part of the ones that don't swoon from his charm, and it's fine. But believe it or not when I tell you his importance to gaming is more substantial than at first glance.
To preface, RPG's were generally obscured by more flashy titles at the time, and though they did have their fair share of fans, it continued to be a substandard amount for quite a while here in the West. Enter Mario RPG; a unique twist on your average Mario adventure at the time, and destined for generous profit given its branding as a Mario game. For some, Mario RPG helped ignite a flame that would soon evolve into a burning passion for the genre, and for others, the game left an impression for being so different than games they were used to. In any case, Mario RPG was helped push the RPG genre into the limelight, and this is part of why I want Geno in Smash; I think Mario RPG games are overdue for a rep in Smash taking into account their sales, and there's really no better place to start imo than Mario RPG, the game that helped set the current RPGs into motion and (this is even coming from someone who finds Partner's in Time better) even the game with the most memorable characters. Geno, the undisputed most beloved of the characters introduced and arguably the character who's the backbone of the story. While you could argue "Mario represents the RPG games well enough", I think that's kind of a disservice to the games since the whole point of the RPG games were to provide a fun and different take on Mario. It doesn't sit right with me knowing that the most commonplace of characters in the franchise (and in gaming as a whole) is representative games that, strip them of their "Mario" status, would probably be unrecognizable to most.
On top of that, Geno is someone I find to have very interesting potential for a moveset. For reference, I suggest the Project M mod on the first page with a fan-created moveset in use. I definitely don't want a carbon copy of what was shown in that video, but I think it sends the message well that Geno has a very large arsenal of moves at his disposal (probably one much greater than you'd expect for a one-off character) that, for the most part, are all practically unheard of in existing movesets.
But anyway, his appeal isn't for everyone, and you don't have to force yourself to find what you consider good in him. These are just some of the reasons I feel the way I do.