Ganon does not hate approaching. He's certainly not good at it, but it's not this all-encompassing weakness.
What is Ganon's all-encompassing weakness is his crappy CQC. How is Mewtwo going to exploit that?
This is the third time in a week a super lightweight character has been said to win against Ganon so here's a rule to live by: If your weight is Olimar or lower and your name is not Rosalina you lose against Ganon. The end.
I can't see MK vs Ganon being that bad, but it's definitely in MK's favor.
As a note, I said that I think the Jigglypuff vs. Ganon matchup is EVEN or better, key word being even, AND I said I don't really know what I'm talking about in that respect.
However, Mewtwo is a whole other can of beans when talking about a lightweight character fighting Ganondorf.
Yes, if you're lightweight you risk dying earlier to Ganondorf, of course. However, here are some things to consider:
Mewtwo does not lose to Ganondorf in range or CQC.
Mewtwo's packs almost as much kill power as Ganondorf AND has an easier time gimping him.
Mewtwo is faster, packs a strong projectile, and severely punishes attempts to get close due to a combination of...everything he does.
It is pretty much a matchup where Mewtwo has the advantage in terms of pretty much all of his tools, but is just held back by size and weight. Ganondorf always, always has a chance to turn it around against Mewtwo but the reverse is also just as true. Mewtwo can juggle Ganondorf, out-space him, out-run him, AND if he really wants to, get up close and personal with the obvious caveat that it's a high-risk, high-reward situation.
A lot of the qualities that make Rosalina a pain for Ganondorf to fight are present in Mewtwo to a lesser extent, and he has a lot of other qualities that are specifically very annoying for a character like Ganondorf to deal with.
Honestly I consider Mewtwo vs. Ganondorf (and vice versa) one of the most fun matchups in the game for precisely these reasons. I think it's one of those weird matchups where both sides are so extreme it more or less amounts to even. Stating that Mewtwo clearly loses to Ganondorf just because of Mewtwo's weight is I think kind of ridiculous.