Would it be wrong to say that the reason FE representation is so hated by Smash fans is probably not because of it's quantity, but how much fan-favourite characters there are? The Legend Of Zelda series is pretty dodgy for representation - three Links, two Zeldas, and a Ganondorf, put in over more unique characters like Skull Kid, Midna, etc. Pokemon, meanwhile, keeps getting marketing representatives rather than fan wanted ones - between Greninja in Sm4sh and it looking like Incineroar for Ultimate, we won't have got a Pokemon not motivated by marketing since 2008, a whole decade. Between Mario, Pokemon, LoZ, and FE - the four largest represented series - only Mario and Fire Emblem really have a varied roster of fighters.
That said, I've never played a Fire Emblem, but I earnestly enjoy seeing it get new representatives. It's such a massive series with a spectrum of protagonists compared to say, Mario, where almost every game has the same cast (most of which are already in the game), and as a result, each new character feels fair.
You're not
wrong, at all. But at the same time, I'd argue that's more of a perception thing than an absolute.
Consider, for example, Roy. He was back as Smash 4 DLC due to fan demand, yes. But that fan demand came because of
Melee. Very few people in the West have ever played the one FE game Roy starred in. It was essentially a Melee fan request, not a FE fan request, and he was only ever in Melee for marketing reasons to start.
Corrin was also put in for marketing reasons, similar to Greninja. Sakurai was given art and gameplay of the character well before release with the intent of putting Corrin in as DLC, meaning that fans never demanded Corrin at all; Corrin was just given to us. It's a unique kit, which is great, but Fates itself didn't win over too many fans that weren't already in love with Awakening-- and Awakening already got Robin and Lucina. With Chrom, that brings the Awakening count to
three out of eight, even though it's only one game in the series that had no direct sequels.
I liken FE representation to LoZ at this point, as a longtime FE fan myself. No, we don't have a bunch of Links, technically, but we
do have seven lords out of eight possible characters, and there are several recognizable characters with better moveset potential, such as Hector, Reinhardt, Mecaiah, Tiki, Tharja, Azura, Ephraim or Anna. Then there are the two fan-favorite characters I often see supported here that would have supported other games besides Awakening, but no less lost to Chrom anyway: Lyn and Black Knight.
I can understand from an outside perspective thinking that FE fans are getting everything they want, but the truth is that FE is a
very large series to represent, and fans of games before the 3DS titles haven't gotten anything new since Brawl. There's a fair amount of FE fans who are just as unhappy with Chrom/Lucina/Corrin as the rest of the Smash community.