I just find it funny how much Mario mains hate Ness.
Anyways, I think the matchup is slightly in Ness' favor, it is at most 60/40. Most people fail to realize that Ganon only needs about 10 hits to be able to kill you. This is how people like DLA are able to do
this. Even the best make mistakes, and Ganon exploits these mistakes well.
I agree with A2ZOMG in that fair isn't that great of a move. It's best uses are defensive retreating and ACing it's first hits for combos. The damage it does is nothing great at all, you're lucky if does more than 5%. The most fair does is interrupt and annoy opponents. Fair of course does this very well on Ganon since Ganon has no tools to deal with a retreating fair.
Fair isn't Ness' only aerial you know. ALL of Ness' aerials are pretty solid aerials. Unless the character has a sword or a crazy bair like DDD, all of Ness' aerials can compete with or outperform other aerials. Ganon's aerials imo are outperformed by Ness' aerials. Ganon's only really good aerial is uair, which I know Ness' dair can trade hits with. So basically in the air Ganon is only at an advantage if Ness is above and in front of him and if Ness is below him (Ganon's dair is also pretty legit, it'll beat our uair if given time to startup).
There are plenty of times to hit with pkf. Nobody plays 100% defense ever in a game, there are chances to use it. Also pkf is pretty safe when spaced well and/or pivoted, so if it's powershielded you will probably be safe against most characters. If pkf is spaced well against Ganon, the worst you'll get is a Wiz Kick which is risky for Ganon since there is a chance Ness can powershield that.
Pkt forces Ganon to approach, and Ness has a lot of options while using his pkt. Even if Ganon knows these options, Ness usually can pick the best option and be safe or successfully offensive. Pkt can lead to pkt2 (offensively or retreating), floorblast (with a offensive or defensive followup), tailwhipping to prevent approach, and basic/complex pkt mindgames like circling, looping, etc., etc.
Fsmash is pretty bad, I won't lie. Brawl just really f'd up on this move, it's near useless. This will hit every once in a bluemoon, but it's sort of safe if spaced well unless it gets powershielded or the character has good oos options to punish it. I agree with you that this move really won't be hitting anyone in a competitve game.