Using Villager and Smashville I get the following examples of his kill power which I compared to Ganondorf which, alright, is an exaggeration. However Mewtwo has a killing projectile, kill throws and better frame data even though Ganon's isn't bad, so he's not as far behind as you'd think.
Shadow Ball is the earliest KOing projectile in the game not counting customs and it travels almost as fast as charge shot, KOing below 100%.
Usmash hits on frame 9 and KOs before 100%.
Dair comes out on frame 15, has a good hitbox and meteors at 40% with no chance of recovery. On the ground it's always a non-meteor so you can't tech it, it can be shorthop autocanceled and it kills at around 130%. It is among the best meteors in the game, along with Ganondorf's and Falcon's dairs.
Fair hits on frame 7 and kills at around 130%. It's easy to land offstage unlike many bairs or dairs and will often net you the kill at 60-80%. On first glance against a CPU who doesn't DI this doesn't look like a big deal, but the angle it sends you at is very diagonal, meaning DI will only make a negative difference most of the time and it will kill just as early when used against an actual player.
Dsmash comes out on frame 21 meaning it's slow, but has a 4 frame release and very low endlag so it's safe to throw out in most situations. Kills at 120%, which again on first glance isn't great when used against a CPU, but the angle is again perfectly diagonal meaning it will KO human players who know how to DI just as early.
And then we have uthrow, killing at 130%. DI matters very little with this one. With moderate rage it will kill as early as 100%. On halberd with moderate rage it will kill at 80%. Mewtwo also has a jab that comboes into grab on seemingly any character, with or without DI, at kill %s. It's not always about how strong your attacks are, it's about how easy it is to land them and how hard it is to DI them.
Mewtwo's pros aren't very straightforward aside from his teleport, airdodges, rolls and Shadow Ball being very useful, but once you look into it more he's a pretty promising character. If he was a lot heavier (like 100+), rage combined with his excellent recovery, range and evasive tools would make him the hardest character in the game to KO, and his uthrow, bthrow and Shadow Ball would kill extremely early. I think 72 is too low, but I now see why anything higher than 85 would've been too much. Stop complaining.