I have more problems with who they chose as clones than the fact that there are clones.
Dr. Mario is just unnecesary. As a character, he's literally just Mario wearing different clothes, and seeing how this game got
two Mario newcomers, it wasn't exactly necesary to give Mario another extra character.
Dark Pit... Honestly, as a character I like him better than Dr. Mario, because even though he looks and fights exactly like Pit even canonically, at least he's a separate entity that's pretty darn important by himself in the game he's from, and that's a lot more than we can say about Dr. Mario. The problem, again, is that I really don't see the need for another Kid Icarus character, as it's a rather small franchise. Two characters were enough and very appreciated.
Lucina? I'm prefectly okay with her, except for one little detail.
They just didn't do enough with her.
Is she faster? Is she stronger? Does she move differently? Does she have something special about her playstyle that makes you control her in a different way as you'd do with Marth?
Naw, she just has no sweetspot to her sword. The one difference is that you just don't have to worry about spacing correctly, but otherwise, she has the exact same playstyle as Marth. Even Roy had more differences to how things worked.
Buuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuut these are just sort-of annoyances. I don't really "hate" them, I just don't think they were good choices or handled well, or that they add many things of value to the roster. Heck, I'll probably still play as them as I would any other character, but pwah.
Take them off the game and the roster is still fantastic, so I can't really be mad. That doesn't mean I have to like them.
Impa isn't important enough to Zelda to get into Smash, and even if she was, she'd probably be too high-priority to make into a clone.
Impa is one of the few characters who gets repeated through several games, even though they're separate stories with "different" characters. Off the top of my head, there's at least five different incarnations of Impa throughout the series. There's
very few characters that are the same in that sense. She rarely actually has a
big role in the game she's from (only exceptions being Ocarina of Time and Skyward Sword) but the fact that she's repeatedly given the "honor" of getting several incarnations alone makes her important to the series. Pretty much the only other characters who get repeated in the same way are the Triforce trio, Vaati, and Tingle.