Not an excuse for having slow attacks while being made of paper mache.
Let's go over all of his "slow" attacks, hm?
Ftilt: The speed difference between this and dtilt is as clear as night and day, but it has more active frames, damage and knockback and it can actually trip at low percents and kill at stubbornly high percents. How about that? It's endlag is also
low enough that it doesn't usually get punished on whiff when spaced right.
Fsmash: A powerhouse move, plain and simple. Though it should be noted that Mewtwo leans back during the startup which
can result in opponents misspacing, like Mario's or Falcon's.
Dsmash: This is his slowest move in startup, but it makes up for it with near nonexistent endlag. If your opponent doesn't
have a swift projectile you can charge this move with impunity when at a relatively safe distance. A great baiting move against
inexperienced opponents like Ganon's Usmash and a surprisingly safe hard read tool against almost anyone.
Bair: It's slow. Too slow to be used consistently against ground opponents or as an aggressive tool, but it's hit area and duration
make it a good complement to the swift shadow claw.
Uair: I used to hate this move. It seemed by all accounts to be a juggling and combo tool but it's startup greatly hindered
it's use in that regard. Then I noticed it's huge horizontal range letting it poke airborne enemies from awkward angles.
Shadow Ball: Extremely powerful when charged, enough to plow through many lesser projectiles would be enough, but once
again the move has deceptively low endlag so it can be used to harass opponents with a small barrage when the situation calls
for it.
Confusion: A command grab with decent range and not too much endlag that also doubles as a reflector. A shame it doesn't reflect projectiles sooner, but at least the reflect duration is nice and meaty.
So when you take a good look at Mewtwo's options, you realize that if the opponent doesn't have the speed to counter-hit (ie interrupt) Mewtwo's moves, he's surprisingly difficult to punish. The only moves that have significant endlag, landing lag aside, are usmash, shadow claw (both of which are swift on startup instead) and fsmash. He's a lightweight bruiser like Zelda, but unlike her he retains enough mobility and attack speed and safety to be relevant in neutral (on top of functional projectiles).
If you are light you have to be fast.
Let's face it, fast characters almost always dominate tier lists in fighters unless other archetypes utterly overpower them somehow.
You could make almost any character tourney viable if they're fast in any measure.