Ness is very good against Falcon, so you have think when fighing him. Here's some notes:
First, Ness doesn't have a ground game against you. Whenever Ness is landing from anything, you should be able to pressure him with a SHFFL. Second, your Nair and Uair, in general, hit too high to be effective. Third, his DjC makes his aerials almost as fast as Fox's, but not nearly as safe. Fourth, Ness's big strength against you is the huge priority and duration on his F-air, and his combos from it. Ness is also going to have alot of trouble breaking your shield, but can otherwise pressure it very well. Also, Ness sucks when he's above you unless he has a stomp qued up already, just Uair if he tries to do anything from above.
So anyway, you don't have alot of options in this match. Your best moves, in order, are grab, knee, low% raptor boost, stomp, nair and bair. You will be spamming knee appropriately, and going for shieldgrabs off of every DjC that fits the bill, as well as spotting grabs as Ness lands from aerials occasionally. Raptor boost is top notch for punishing missed SHFFLs, follow with a Uair for teh combos and best results. You can also fly around with Nairs; you want to avoid anything but dash-jumped nairs because sparkles eat them up. So that's your basic strategy: Pressure with Knee, get grabs (u/d-throw uair/knee), do some flying stomps and nairs, and punish whiffed SHFFLs with raptor boosts, grabs, and knees.
Ness best bet against you is to space your knees so he goes just over them and comes down with the DjC aerial to punish, or just shieldgrab/shieldhop fair alot. Ness can CT Falcon with U-air mixed in until about 50 or 60%, and after that Uthrow->Fair/Bair until very high percents, and after that just Bthrow you well off the stage. So if the Ness is being all sheild/fair-campy, use a mix of grabs and stomps, and that'll shut him down.
Aggro-Ness is the next stage of the mixup. Ness will attempt to DjC Fair->grab at most percents, because the early hits of the sparkles have fairly fixed knockback and are easy to L-cancel and grab off of. As discussed above, shield-grab him or use Raptor boost to punish.
Pressure-Ness is the hardest to deal with, because it's a very safe style of play for Ness. It basically involves an aggressive Ness that uses his Fair and Bair to harass you, but doesn't land in range of a shieldgrab, and doesn't press his luck, and exploits your knee/stomp counters and your other mistakes with grabs and shield-hopped nairs, allowing him to build damage and apply mindgames. The Fair is a mindgameable attack, because you can SHFFL it, DjC it, dash jump it and pull back, do a double fair, or go through a shield and land behind CF. But anyway, it doesn't matter, because sheild-hopped stomps and Nairs can bring the match back into your control and force him to go aggressive or defensive; make sure to stay mobile and use platforms to your advantage so he can't stay on top of you and space you.
For recovery, sweetspot or go well over, and be prepared to tech a bat or Meteor Cancel a Dair.
