So I'm no expert but this is my take.
You will body Falcon if you can get in, you have reliable F-Air strings that put him into an edgeguarding postition, Falcon has an extremely linear recovery that you can abuse with yo-yo, pretty much any of your aerials, Pk Fire, etc. Falcon lacks spacing tools other than his back air and Nair 1's, and PSI Magnet combos are so easy considering Falcon is a fast-falling character with a weight of 104, and Falcon does not like PK Fire at all, not to mention that it's easy to juggle him either with Up-Air or Pk Thunder, and if he gets close you can always try to launch yourself with Pk Thunder.
On the other hand, Falcon has some good combo tools, has a solid damage output, and is a fast rush-down character that likes to combo. As a Ness player, your recovery is not the best, while you may have the best airdodge in the game, that can't always help, and Falcon can have a heck of a time edgeguarding you. Another thing is that Ness is not the heaviest character, add that to his recovery, can lead you dying at very low percents.
What you have to do is understand what Falcon is trying to do, be aware of the risks his combos and off-stage game present, and outrange him. SPace him out with aerials, bait him with PSI Magent into whatever your heart is content with, and force him off the stage, and gimp him. It can be difficult as Falcon can overwhelm you, but you should be able to do it.