Eh. It's not that noticeable. It really just means she can't outright destroy bombs and can damage Red Pikmin, which are both still pretty niche interactions. Also extremely niche, but her Charge Shot hits lower, allowing the uncharged version to hit Pichu sized characters more easily.
That's more noticeable than Richter, imo. But regardless, she does have more differences.
I think the main thing is that people just don't like standard Echo Fighters, and want to know how far they can push the concept because what they seemingly could be is something that they'd want.
It's established that they need to have the same proportions, except there is some leeway because this isn't true of Marth and Lucina. So then, Black Shadow might work should giving him Captain Falcon's proportions not cause things to look weird. Oh, and the height difference thing with Marth and Lucina is very strange, as since Lucina was originally a costume, she had to have been shrunken down after they gave her her own slot. It's not that impactful a change, and everything scales based on characters' models so did they change it tom be consistant with the others like you said or were they just correcting an accident they didn't notice in SSB4's development?
Lucina was only different from Marth in height in SSB4. They changed it to so they're pretty much the same in Ultimate specifically. This may simply just be because she became an Echo Fighter and was adhering to a new rule.
There's also weird hypotheticals with Villager and Isabelle. Isabelle can't be an Echo Fighter because her perportions are very different compared to Villager, but that's only because Villager uses his design from Animal Crossing: Wild World, while Isabelle uses her design from Animal Crossing: New Leaf. If Villager was also from New Leaf would Isabelle be an Echo Fighter? Their proportions aren't exactly the same in that game, but as we've established, they just need to be similar enough to be made the same without issue. If they did become Echo Fighters, would Isabelle be less different from Villager than she is now?
The bodyshape was part of it, but Sakurai also noted Isabelle needed new moves to more accurately portray her. So her chances of being an Echo Fighter were slim even if Villager had a new model.
How many changes can you make to a character's moveset before it becomes too much work for an Echo Fighter? Most of them say basically none at all, having at best some different, mostly aesthetic properties on some of their moves. Chrom and Lucina have sweeping numbers changes to their attacks, and Chrom even has a new up special. This is probably not applicable to all Echo Fighters though, so what else is there? Well Ken has several new moves, and a few movement property changes that allow him a more aggressive variation on Ryu's play style, but it's so out of the norm that people often dismiss it. Then there's Dark Pit who has two variations on Pit's Special Moves that are impactful, but it doesn't change his overall playstyle much, and people aren't super happy with it.
That's why it's confusing. We know the hard rule is "same bodyshape", but that's just at best what we're given. We've seen, as I said, that they all have the same weight, and it's worth noting the attributes are generally not very different(Ken has mildly different ones). The other thing about Ken is that despite this, he and Ryu aren't that far off competitively from each other. This is also why people can understand him staying at that label, since it's clear that they're still pretty similar. Dr. Mario only looks similar, but his changes make him nightmarishly different from Mario, being worlds apart. Thus, it makes sense why he wasn't one. There's also the factor he already was in previous games like Melee, but was also planned for Brawl. So his labeling is likely just a Veteran thing. That said, Ken easily could've been a non-Echo too. The rest are not nearly as massive as Ken. Lucina's thing is that her animations are mostly the same, it's just they didn't give her the same mechanic(Chrom is this too, though he uses a different way to hold his sword, while still sharing Roy's moveset, but also has a new move).
The result is people suggesting Echo Fighters to add characters that woudln't get in otherwise, but wanting them to have actually impactful gameplay changes, and whether or not those changes can be made is pretty hotly debated because half of the people see the opportunity to add more Kens and the other half only see more Daisys.
Oh, I agree Daisy is ridiculously bad at this. I like to put 'em up like this in terms of uniqueness;
Ken > Chrom > Lucina > Dark Pit > Dark Samus > Richter > Daisy. I feel that Dark Samus is okay since she has some minor enough differences that can affect gameplay when it comes to the "least possible". Richter doesn't need much more, and Daisy is just... wow.
We'll have to get used to Echo Fighters sometimes not having truly impactful gameplay, though. Like, Dark Samus isn't bad(though not great) persay, because they at least gave her some changes. This isn't a bit ridiculous like Richter having one change, and Daisy barely having anything(which ironically makes her the highest tier among the Echo Fighters, lol). Like, I don't think her having worse Turnips like the accidental patch did is enough. Just a tad more would do it.
That said, nothing wrong with wanting Lucina levels either(she has some impactful differences, while still keeping the same point behind her). I think what's also confusing is that an Echo can have the same mechanics(Ken's mechanics are basically Roy's, but he has different move commands to match his own personal gameplay) to differentiate them, or not have the same mechanic at all(Lucina) but still stand out. So it's a matter of "how they approach it". But yeah, we just lack details beyond one hard rule.