This is way late, as per usual for me, but I figured I'd put my two cents about Fire Emblem representation out there into the aether, particularly because I don't think I've seen anyone communicate it before. Also before I say anything I just want to note that I don't have anything against Fire Emblem. I do think it faces a somewhat unique issue when it comes to Smash representation in particular though.
Anyway I think that the problem with FE representation, and why it seems to cause such a schism, is that characters from Fire Emblem are difficult to represent in a way that feels fully unique to people who aren't familiar with the franchise (myself included, though I do hope to change that soon). And what I mean by that is that personality play a very large role in how Fire Emblem characters are perceived. To someone who only plays Smash, there may as well not really be a difference between Marth, Lucina, Roy, and now Chrom apart from gender and the odd flame effect (keeping in mind that I imagine a lot of casual fans may not even know about the tipper mechanic). Those fans know nothing about what the character has done, what their personality is like, what their history is and that seems to be a huge part of what makes the characters feel important and unique. But to everyone who has played the games, the characters are obviously different, because they have context. This is basically a long winded way of saying they care.
Take a different game with a ton of characters like Pokemon for example, and I think the differences are pretty apparent. Even if you don't know much of anything about Pokemon, every character from Pokemon will have a wildly different silhouette and their moves will be wildly different because of how typings work, so even though they don't have a particular personality to them the casual fan immediately recognizes why those characters are different and potentially deserving of a slot. That's an extreme example, but I think it fits. Especially given that Smash Bros as a series is a game that includes characters that in general a extremely visually distinct, often even when from the same series. Take Star Fox for another example, and even though every character started as a sort of semi clone and may not always feel super distinct mechanically, you can still differentiate very very well on a surface level because one is a bird and one is a fox and one is a wolf. Fire Emblem on the other hand, every character is a human, with similar-ish builds, in the same anime-ish style, and many use swords. And so, to people who lack context for the characters, they feel much more similar, they begin to blend together, and thus begin to feel less individually necessary.
This is a huge reason that people advocate for new characters to have polearms or axes or magic, even beyond just the actual gameplay that might entail (because let's be real an axe user could really end up playing exactly like Ike already does). It's a very simple way to communicate that this character is different. This character has something going on, at least on a surface level, that other Fire Emblem characters don't. It makes them feel meaningfully different, even if perhaps they really aren't all that mechanically distinct when you break it down.
I think this is a problem people would also have with other series like say Mother if we were to continue to get characters like Ninten, Paula and so on. But I think this is a huge problem for FE in particular because it's a big series and there are just so many potential characters to add that in general don't recur with a visual style that is very similar between characters (note this is not a bad thing overall). Add to that that the most popular choices seem to be lords (which correct me if I'm wrong tend to default to swords?) and people who aren't familiar with the characters start to groan. This seems to be something that could theoretically be remedied by different character selections (maybe a sword user, a lance user, an axe user, a mage, and a dragon), but we all saw what happened when Chrom wasn't included in Smash 4 so it doesn't seem like there's a great way to keep fans of FE happy while also including characters that feel meaningfully different to casual fans.
But the TLDR of this, for those that really enjoy Fire Emblem and who advocate for more Fire Emblem characters, is to just keep in mind how much of what you love about a character requires context to understand. What makes FE characters different is simply less immediately apparent to people who don't already know them. Believe me as someone who wants Isaac in this game, the number of people who assume he would be just like a Fire Emblem character is astounding and he isn't even from the same game. This is frustrating, but also understandable. And unfortunately Sakurai has to keep these people in mind as well, because ultimately he needs to make the game as broadly appealing as possible.
EDIT - this thread grew two pages while I was typing this monstrosity...