As far as Fire Emblem characters are concerned, Roy is pretty much the only one whose popularity mostly stems from Smash. Every other one chosen for Smash was popular long before they were picked. And a lot of that stems from the fact that the Lords of each game tend to be very popular.
Marth is a bit of an icon in Japan due to being the first Lord, and was one of the biggest requests on the Melee poll as a result.
Ike was a huge request back in the Brawl days, especially from western Fire Emblem fans who really took to him.
Chrom was not only a huge request for Smash 4 (with the reaction to him missing the boat being the catalyst behind him getting into Ultimate later on), but was also the most popular male Awakening character on IS's official poll, while Lucina was the most popular on the female side. Robin also did very well there, taking second in the female division and third in the men's division (for those wondering, the character between Chrom and Male Robin was Owain lol).
Female Corrin was the most popular female Fates character on IS's official Fates poll. Male Corrin didn't do nearly as well, but the disparity between the popularity of the two Corrins is fairly well documented regardless. Smash really is the outlier with having Male Corrin be the default lol.
Byleth...is fairly popular, but comes with the caveat of being significantly less popular than Edelgard, Dimitri, and Claude, the three of whom are, honestly, probably the most popular characters in the entire franchise. Byleth gets hurt in comparison due to being a mostly silent protagonist, and it's pretty easy to see IS not doing something like that again given the circumstances lol. So either way, Byleth makes sense as a neutral choice if nothing else.
Roy, therefore, is the only one whose popularity wasn't a thing before Smash...on account of the fact that The Binding Blade didn't release until after Melee. But even then, that doesn't make his popularity lesser or anything. Popularity is still popularity, regardless of the source. I mean, look at Pikachu. Most of its popularity undeniably comes from the anime and merchandising more than the actual games. But nobody would dare call Pikachu's popularity into question. Obviously Roy doesn't reach nearly the same heights in popularity as Pikachu, but the principle still stands.