That last sentence could be applied to Ganondorf, Zelda, Roy, Marth, Ryu, Bowser, Charizard, Ike, Samus, Dr. Mario, and a bunch of other characters if not the entire cast.
In thinking about this, though, how many characters actually have as complete a KO package as Mewtwo?
Ganondorf is filled to the brim with KO moves, but lacks a real kill throw, and doesn't have a projectile.
Zelda has a lot of strong attacks (especially if sweetspotted), but lacks a truly powerful deadly projectile.
Roy can combo off of throws for a kill, but doesn't really have a kill throw. Also the issue of sweet spots is much more pronounced of course.
Marth is somewhat similar, but that early tipper isn't to be underestimated.
Ryu has THE Kill move in True Shoryuken, and a ton of nice powerful smash attacks, but his most frightening KO power is concentrated on that one kill move.
Bowser, Charizard can kill off of a lot, including off of grabs, smashes (of course) and even tilts, but lacks a killing projectile.
Ike has power of course and sort of has a kill throw.
Samus has the projectile and some decent kill power, but her kill options aren't that diverse.
Dr. Mario's back throw is a bit weak, has no killing projectile, and range of course is an issue.
None of these things the characters lack make them bad or good characters inherently, but then you look at just the ways Mewtwo can land an early or medium-damage kill or turn things around from a certain position, he kind of has a lot.
Forward and Down Smash kill at reasonable percents. Up Smash is among the strongest in the game.
TWO Kill throws, including the strongest up throw.
A deadly projectile that also creates opportunities for followups.
A fairly quick and powerful spike, combined with a strong recovery.
The only thing Mewtwo lacks when it comes to kill power is strong tilts.