Hi, welcome to the 64 section
Others will probably give you more detailed explanation for specific cases, but in general, I'd say :
1) You guys should work on your z-cancels and techs. I'm curious as to why you missed so many techs, do you know you don't need to time z to tech in this game (ie you can simply smash the z button before touching the ground)? Concerning z-cancel, fortunately Ness' uair and dair don't need z-cancels (they always autocancel), so you only need to z-cancel other aerials.
2)You should also refrain from doing so many random b moves (both neutral b and up b) when it's not safe/not the best option. You could have got heavily punished a lot of times with those upb's.
3) Try also to avoid shielding aerial attacks, as the huge amount of shieldstun is generally resulting in a free grab for the attacker.
4) Concerning edgeguards, down smash is an easy and effective tool, try it!
5) Fair isn't a really good approach move, try Bair'ing more. Also because the extended DJC goes further with bair, so you have more flexibility (you seem to have the basics of DJC down, so try to be at ease with mixing between fast/almost standing DJC's and horizontally extended DJC's).
The problem is that it's difficult for you to notice what is a mistake if you're not punished for it (as would happen if you played someone significantly better).
