I know this is a late reply, but ok genius, let's look back at Melee. Pikachu was considered a bad character at the start of that game's existence, just like he was at the start of the 3DS days like you said. But let's see, who else was considered a bad character at the start of Melee's existence. Hmm....who was that again?......hmm......OH THAT'S RIGHT, MEWTWO WAS! Well what do you know?! And how have both of those characters turned out in Melee? Pikachu is now high tier, and Mewtwo is STILL near the bottom of tier list, still not viable, and still never will be viable. And the same thing is going to happen with Mewtwo in Smash 4 as well if the way he is right now.
So yeah, you can probably trick someone else with your BS argument of "oh, well Pikachu was considered bad but now he's good, so just wait for the metagame to develop and he'll be fine", but that's not going to work on me. Yes, all characters have weaknesses, but you have to understand that most characters do not have the kind of crippling weaknesses that Mewtwo has, and those that do are not going to be viable characters either. It's not that easy to just say "well all characters have weaknesses, just turn those into possibilities". It doesn't matter if it is Melee or Smash 4, that just not always going to happen.
And "instead of complaining, get good"? YOU CAN'T! Not Pikachu level good, oh no. There is no way in hell that will ever happen with the way he is right now. I'm complaining, like maybe people are, so that maybe these inescapable flaws will be fixed in future patches, because that is the only way this character has a chance. Even if you fix these flaws, he is still going to be a character that you have to really work hard with to get good, but at least if his major flaws are patched in any way, your hard work can actually pay off, because right now, no matter how hard you work with him, Mewtwo will never be a viable threat.
And personally, with him being this light and this big of a target, he is just not fun to play as. So even if he has a ounce of viability in him the way he is right now, I'm not going to be wasting my time trying to get good with a character that I don't enjoy playing as. The character that I wanted back was Mewtwo, and this lame, glass cannon piece of **** is not Mewtwo. So I will never be happy about it, sorry pal