My best friend mains Ness so I have pretty much experience against Ness. I main ZSS and Falcon and I have to say Ness is the s***. His aerial attacks easily play out Falcons nair or bair. So you have to read the pkfire, jump or roll through it and try to game him with grab combos - but watch out in the air. If he gives you a uair or backair, damn you're done.
If you stay in the pkfire you will surely get 30-40% dmg - if that is worth for some jabs for you then stay in it. I just roll back and counter on his approach with a downtilt. The downtilts knocks him very far away so I have some time to think about what to do next.
Indeed he is easy to stomp with dair, try to get used to their airdodges and learn to punish them. I'm not very good in that yet, but if you hit a punish with Falcon, he can be killed easy.
Also, what I trained vs CPUs is the downair vs his pk thunder recovery. I have arond 70-80% hitchance with a dair. Unfortunately Ness's second jump is as good that he barely has to use it.
I think backair is the key to kill him. It's fast and the hitbox is very big, if you land some of them you can kill him good and save.
Try to stay out of pkfire, and learn to avoid the pkthunder while he chases you with it.
By the way, I am so waiting for bthrow nerf. This throw is the most ridiculous thing in smash since ever. Kills at 80% with rage. Yes I'm salty. I always say that I don't respect it if he kills me with that. That makes him salty too.
To be honest it doesn't take that much skill to just go for the grab if the opponent reaches 80% upwards.