Mewtwo, and Samus, and WFT, and Robin, and Lucario somewhat, are Charge Shot characters. They live and die by these moves. Charge shots are gigantic boons in FFAs, so naturally they need drawbacks for balance. Which, one way or another, means taking away CQC abilities. Lucario is weird but has aura though... Actually, how stupid his aura is makes sense in a FFA environment.
Anyway, until you remove the Charge Shot, these characters will be severely held back by the moves. This is why Phantoms aren't storable, as Zelda would have to be even worse in 1v1s to compensate.
This seems to be the leading defense (explanation?) for why these characters are so weak. But the problem with this idea is that it's still dreadful design even if that were the case. There are worse moves to be caught with in FFA than Samus' whack charge shot, it really isn't that strong, lets be completely honest here.
I mean...it's a charge shot. It would be understandable if Samus could reliably combo into the attack, but she can't. Or if it killed
reliably sometime before 130%, but it doesn't. DK has no lack of power for being able to store his punch, Shiek loses nothing from holding needles, Mario loses nothing from charging Fludd, and in most cases (barring shiek), these moves are essentially worthless UNLESS they're fully charged. Nor do Rob, Wario, or Robin, and those are both killmoves. And Villager/G&W lose nothing from being able to store ridiculous amounts of MULTIPLIED killpower behind a 4 frame activation and/or down throw true combo. Where is Pac Man's lack of 1v1 utiltiy with his charge attack removed?
Samus is just badly designed, whether we're talking 1v1, FFA or 8-player smash.
And don't even get me started on Zelda. She's only a bad character because she's designed poorly, plain and simple. They removed Shiek from her and gave her no compensation for it. You have to be
sleeping to even be hit by Dins Fire, let alone be killed by it because its sourspot is absolutely massive. Phantom? What is it even for? Who's going to be hit by that thing? It's the most telegraphed attack in the game, it's incredibly slow, and having it on the field gives Zelda no tactical advantage whatsoever because she is NO threat to you behind it.
High risk- High reward characters have it hard the meta game. The only ones who are good seek to be Falcon and to an extent Lucario and DK.
The reason why Doc isn't used is the same reason why Lucina, Lukas, and Falco aren't being used much, they are just outclassed by the character they are a clone off. In the case of Lukas and Dr. Mario, it seems like they are almost halfway decent, but everyone just picks Ness/Mario over them. Lucina and Falco seem to just be bad though.
Lucario can be decent...sometimes, but generally speaking he is a pretty worthless character unless you're at 120%, at which point he begins to become quite terrifying.
I'd hardly call Falco a clone of Fox in this game, but Falco is one projectile buff away from being a pretty decent character. That laser is just, like, really bad though. Another case of nerfing an integral move and compensating in all the wrong ways.
Lucina is only "bad" if you think Marth is bad, and that goes the same with Dr. Mario....sort of. Lucina being Fsmash the character really isn't that bad of an affliction (considering her Fsmash is damn good) when Marth by comparison is....well, Tippered-Fsmash the Character.
Unfortunately, Dr.Mario doesn't share the same mechanical similarities that Marth/Lucina do, since his mobility and recovery are notably worse than Mario's, and so he'll actually suffer in ways that his clone doesn't. Lucina does not have that issue. Whereas she's (slightly) less safe on block, i'll take her Uptilt over Marth's occasional critical strike when your bad opponent doesn't respect the fact Fsmash Tipper will kill you at 60%. Then again, "kill at 60%". sooooo...
But is Doc VIABLE? Unless people can flowchart ways to gimp him, i'd say honestly say he is, off the basic principle that, well, he's Mario....with an even dumber UpSmash and some scary strength. Unless the Doc boards are having trouble being gimped, I don't think he's that far away from Mario. What he lacks in whatever mobility he loses, he still gains in having by default better move properties than
more than half the cast.