Despite Ness being a good character, his neutral is what prevents him from being top 5 IMO. This is actually the hardest part of playing Ness (even more than recovery to me), and a lot of people fail to realize that Ness has this glaring flaw.
Ness doesn't have huge range aerials that are safe on landing (looking at you Sheik and ZSS). He's also got a really low dash speed, and no real projectile to cover himself with (PKF isn't a real projectile, it's a combo starting tool).
So the best you can do is adjust to your opponent and take advantage of the moves they don't have to help their neutral.
Ness' best defensive tool is nair. Use this on characters who like to take it to the air and outspeed/prioritize you. At best, you get a clean hit, at worst, you get a trade. Ness nair trades rather well and beats a surprising amount of stuff. If they're going super aggro, shielding into nair OoS is a great option. Sometimes, you just have to be okay with trading aerials instead of just outright beating the opponent. This thing also eats through a ton of projectiles like stun guns, Pikmin, and slingshots. Also use nair as a mixup if the opponent is spot dodging your grabs.
Fairs can sometimes work, but I feel like Ness players overrate it. It has a lot of range and priority, but just be careful not to trade or get hit out of it. It's not the fastest aerial, and it's incredibly weak, so if you trade with it, you lose.
Dash attack is an underrated neutral move IMO that can be useful when used in moderation. The range is really long, and against people who like to stick to the ground and fake you out, this can really catch them by surprise at times. It pops up opponents for juggles at lower percents, and sets up for reads at higher ones. It can also beat rolls due to its multiple hits. But remember, don't get predictable and obvious with it. Pull this out every now and then, but don't abuse it.
Bair has a surprising amount of pushback on shield (when sweetspotted), and really low landing lag, so this is the safest aerial against shields (technically uair has less landing lag, but bair pushes shields back more). I recommend using this on people who turtle a lot in their shields, and aren't fast enough to punish it when spaced well. Characters like Luigi and Robin will have a hard time punishing this. In the air, this move also has a good amount of priority, and hits like a truck.
Magnet into magnet cancels for obvious projectile users, easy enough.
Don't forget the basics too. Tomahawks and running into powershields will be useful for people who shield a lot or use projectiles to wall you out.
Lastly, I want to discuss a tech that I've been talking about with other Ness mains. SH air dodging into aerials is a neat little trick that can get you through attacks, and punish them during the end lag. Ness' air dodge ends right before he touches the ground, giving you just enough time to nair or uair before landing (I recommend uair). You can use this to charge right into attacks and the retaliate. Just make sure you air dodge right at the start of the SH. And as usual, it's still punishable, so use it wisely.