This is what I think about Ganon in Sm4sh.
One other thing that can really make a character good is a kill throw. If you can rack up percent, your neutral can be bad, you can have laggy moves (to an extent), but if you get a grab at high percent, it's stock.
Shiek gets past that by having amazing frame data and kill setups. R.O.B. has a really strong Dthrow > Uair. Ness has an amazingly strong Bthrow, which ends up netting him like 50% of his kills. These features really help their characters, and unfortunately, I think that Mr.
doesn't really have any of these options for kills.
One reason I would say Ganon is low threat wise is because he doesn't have a really strong killthrow. (He's the king of evil, why can't he be stronger than Ness?) If I requested a change for Ganon, I would want his Back and Foward throw to have more knockback.
As @
Trifroze
mentioned, I agree that a character needs a good sub-8-frame get-off-me attack, and I think that Ganon doesn't really have that. His Nair is fast and so is his Uair, but his hurtbox is so big he can just get combo'ed to high percent. He doesn't have a Yoshi or Luigi Nair. For him to be more of a threat, he needs to have better defensive options.
Now to argue in Ganon's defense. He has a good jab (Frame 8) and a good Ftilt that is surprisingly safe in the neutral, a lagless Usmash, and his Uair auto-cancels, which means it's relatively safe on shield. His punish game is great, simply because he does so much damage, and his smashes kill really early. I'm really glad the gave him a combo throw, which means a grab does that much more percent.
These benefits above are what makes the character playable imo. Ganon really just has to be regarded with caution. If you play against him right, you shouldn't lose with a High or Top tier character. He gets combo'ed easily, and his neutral is bad, which means easy grabs which means lots of damage. He can be gimped easily and he doesn't have many follow-ups to aerials.
I would put Ganon after Mewtwo threat-wise.