I play this matchup a lot, since one of the people I brawl the most mains Ganon.
In the air, Ganon's Uair is amazing, Ness has a hard time getting around it, and off stage it can stage spike. The huge problem in the air for Ganon is that his horizontal attacks (Bair and Fair) are completely out ranged by Ness, and near useless in this matchup. Dair is some what situational, but racks up a ton of damage, and kills Ness is he's on the ground. Wizard Foot is a huge annoyance at first, but is actually very punishable with almost all of Ness' aerials, and PK Fire and Thunder. You just have to be very quick about it, Ness wins by a somewhat large degree in the air.
On the ground, Ganon's choke and wizard foot are horrible for Ness, but thankfully are both avoided by a spot dodge, so it not really a on which to dodge. Choke>Dtilt is a combo on Ness, and if you predict Ness' movements, you can choke Ness more than once thanks to his long getting up attack. Ganon easily can escape the edge of PK Fire and the first hit, so PK Fire is only useful when Ganon is approaching with choke, or just running towards you. Ganon can also escape PK Fire using his Wizard's foot, bewary of this. Using Dash Attack isn't a good approach against Ganon, because of Wizard's foot, so using aerials near the ground is a better approach, PK Jump is somewhat effective too. Ganon's jab is quick and has decent range, but it only has one hit, and doesn't combo into much that isn't avoidable or easy to shield. Yo-yo also helps against most of Ganon's attacks on the ground because of it's charging hitbox, just don't become predictable because UpSmash and DSmash are easy for Ganon to punish. It's possibly even IMO.
Off stage, watch out for Ganon's Uair and Side-B. Uair has large range and can stage spike, and Side-B=suicide. Ganon's recovery is very easy Pillar spike, PK Flash (if he's covering from a afar), Yo-yo Spike, Ledge Whip, and just plain Dair. If Ness is using PK Thunder too close to stage (which is never a smart idea unless you lost your 2nd jump.) Then he will easily be KOed by a Dair at higher percents, sometimes a side-B if the Ganon is willing to be risky. Recovering near the ledge is hard against Ganon, because you can easily be spiked. Horizontal recovery is easy however. Ness' wins off-stage.
In the end, it's around 60-40 to 65-35 IMO.