Except I DO care about the character, in fact, just as much as I care about their moveset.
And the way to show a love and appreciation for a character is with something true to the character and shows off their differences. A moveset is more than just a generic playstyle that one might slap onto a character if you can halfheartedly justify it. It's supposed to represent them, the essence of their character, and their transition into a fighting environment, which is why even unique movesets could poorly represent a character, like IMO Shulk or ZSS.
And for some clones, it would make sense they would fight similarly like Dark Pit being a literal clone or Falco probably being trained the same as Fox. However, I'd disagree they'd make sense for some other characters. Roy wouldn't fight like Marth, Lucina wouldn't fight like Marth, Chrom wouldn't fight like Marth. Dark Samus wouldn't fight like Samus.
Even for characters you could justify as fighting the same, there are limitless ways to differentiate them. For example, Hyrule Warriors differentiated 3 different Links with 3 completely different fighting styles that all match their games more than copying the main Link or how Dragonball FighterZ had 4 different Gokus and made them play completely differently.
Differentiating movesets is certainly the preferred way to go, but because of how they tend to plan characters out and what movesets to create in advance, including someone as a clone is not necessarily a bad thing but a chance for a fighter who might not have been considered at all otherwise. You also can't randomly choose who will become a clone either, so in way, their status as a copy character needs to make sense. For most of the examples at least.
They bothered with the echoes because they were convenient to create, and I believe that these characters have a lot of elements in their backgrounds to justify why they are like that. In addressing Lucina, her being a Marth alt is actually a mythology gag from Fire Emblem Awakening. In that game, recruitable Marth units are actually head swaps of Lucina's character model, basically making Marth a Lucina clone. It helps as well that she was impersonating as him in the beginning of the game. Regarding Roy and Chrom, being different people you can definitively say that they wouldn't fight like Marth, but the issue that FE characters from the GBA era have is that they don't have much of a movepool to pull from to translate into a fighting game. Add that Roy was just a late addition, his moveset being the way it is isn't really unjustified. At least they tweaked his stats and move animations while giving him his critical attack as his FS, so he stands better on his own now compared to when he first debuted. In Chrom's case, Robin was chosen over him and they never really materialized the idea of a unique moveset for him.
As for Dark Samus, she mimics most of Samus's abilities. As we know, this is because she was born out of her genetic material and the Phazon Suit she was wearing. I'm aware she has unique moves of her own but she still displays the basic Power Suit abilities like missiles, power missiles, morph ball and boost ball. Her Phazon Spreader is even referred as an altered version of the Power Beam in-universe. So resorting to the same moves Samus has in Smash like Charge Shot, Bombs, Grapple and Screw Attack is by no means a stretch. In universe, she is more or less a phazon fueled copy of her.
I would argue about the Links playstyles in HW matching their canon material. HW exaggerates every aspect about every single character and takes several liberties as well. Not that there's anything wrong with that, but their Smash incarnations are actually more faithful to the source. That's doesn't mean that their HW counterparts are less of a blast to play as. Since Link was redesigned for Ultimate, I would say that he is different enough from the other Links to justify their presence.