I think at least within the smash speculation scene there is a bit of a reluctance to support what is seen as possibly a "fad" character or a promotional pick (there's a reason why Corrin is not liked), as well as not having a pulse on what is liked casually. Like for example before I became the
#1 Oatchi stan, during smash ultimate speculation I was a big supporter for another funny yellow dog in the form of Isabelle
.
Despite it being seen as an obvious choice now, Getting people to acknowledge Isabelle as even a character candidate was like pulling teeth even when people were still making rosters for ultimate with the expectation of 15+ newcomers at base game. Before Isabelle was revealed most discussions about her were arguments over that she was a pacifist and wouldn't fight.
I don't think Isabelle was seen as a fad or a promotional pick, those are like Pokemon or FE characters or new series. I think she was just seen as a character of highly contested feasibility.
And I actually think there's a great influx of support a lot of potential fad/promotional picks receive, it's just that nine times out of ten they wash away once that character recedes from their moment in the limelight.
Like, Noah is going to receiving support right up until the point the next Xenoblade is revealed - assuming its timeframe is somewhat compatible with Smash. But if he doesn't make it, and there's a new Xenoblade, he's basically done in the speculation scene.
Also the reason Corrin isn't liked is mostly because FE was pretty highly represented at the point that character showed up, and no one really asked for them. Plus they were revealed at a bad time, in the lead-up to the "ballot winner". If you rewind - Ike, who was also the most recent Lord (at least at the time of selection) and could've been seen as a "fad" or promo pick
was liked. Or before that, Roy - who no one even knew, and was a clone and promotional, but didn't come from an "oversaturated" series. Or how Rex & Pyra were fairly popular, albeit divisive (though mostly due to their designs).
I guess a reason is that in the past there has been many newer characters that seemed like they had moderate support and a viewed decent chance but then don't make it as a character and their support erodes as their newness fades. Like for example Wonder Red is one, he was constantly on every hypothetical roster for base game Ultimate and even Smash 4 DLC, but you don't hear much about them anymore. I think a lot of people are reluctant to spend their energy on supporting new characters as they don't want to bet on the wrong horse, especially when the "same 5 characters they grew up with" have a longstanding community around them and they can spend time with likeminded people.
Well yeah, there obviously are characters the fanbase gravitates to over others, but most of thise who can maintain their support across games usually have some sort of reasoning past the good luck of being in the fanbase's favor. Because sound reasoning begets expectation, which feeds popularity.
It's hard to fuel a fire for a character when they come from a series of middling to low success and then stop getting games. Expectation and desire work on a feedback loop, and there's no real reason to expect characters like Wonder Red. Even back then it was idealistic; a recent IP still requires success. That's why we could get a Xenoblade, but a W101 was not in the same boat.
I mean if you look at characters like Geno, who has popularity regardless of his honestly pretty low overall standing, simply because people like him - that's basically the sole exception. And he's not in yet. Or if you look at Isaac, who is also an exception insofar as being a dormant series that has maintained demand, at least those games were successful. And he, likewise, is also not in.
It's not that these characters can't get in, it's that when even more qualified options still have people pushing for them, the ones with weaker resumes are going to find keeping hope alive that much more of an uphill battle. Which isn't without reason... there's basically no precedent for inclusions like these yet.