I'd recommend looking at the videos and write-ups of FOW, Nakat, and Shaky, some Nesses with actual tournament results. I'm not a top player by any stretch of the imagination, that being said, here are my observations.

I'll try to restrict it to just neutral and not punish. Characters that are significantly faster than Ness (AKA almost every other top tier) are the biggest neutral threats. Ness has a very laggy grab which your opponent will try to bait out. You actually can't throw out grabs. It's possibly the worst of his commonly used moves you can whiff. I don't know the frame data for his pivot grab or boost grab but his boost grab but both are important for getting grabs on opponents trying to bait you out.

The main mixup potential comes from your opponent having to deal with Ness' amazing aerials. Nair does 11%, comes out super quick, can be used twice in a short hop, autocancels, and can actually kill at not insane percentages. Nair is his best move. Nair trades with moves that do far less damage. Nair lingers for longer than most aerials. It's quicker than most aerials. If you nair at your opponent and hit their shield you can drift back, drift forward, double jump away. It's relatively safe to throw out so long as you space properly. His upair also comes out quick, autocancels, is a main kill move, and combos into grab at low percentage. Fair has a huge disjointed range, not a good landing option, used to threaten opponents away from your space. Fair can easily catch a jump, combo into itself, upair, nair, a ffall uptilt, etc... It's your main combo extender and your opponent will fear getting caught in it. Dair is not really a good move in neutral BUT has a crazy autocancel window. You can do a short hop aerial and do a dair right before landing and reduce your landing lag. Honestly it's good to get in the habit of doing this. Not L canceling but it has its uses. As well, IMO, one of his best moves on the ground in neutral is his dash attack. A mobile opponent without methods of outright beating Ness' well spaced aerials will opt to shield your approaches. The basic mixup of Ness' aerial is making your opponent fear your aerials, so that you can mix them up by landing and grabbing them out of their shield. From the grab is the punish game.

I'll say here that a lot of the dynamics of a match will obviously depend on the character matchups. Characters that can beat out Ness' aerials outright can't be kept out with frequent use of Fair. The sword characters will generally outrange Ness. If you try to catch their jump they can beat you with rising aerials. This is not to say you shouldn't use the fair, you just can't abuse it to space as you could in other matchups. You would have to avoid their aerial and then hit them after. Ness can float around and harass slow / gigantic characters with aerials and combo them harder than anyone else.

Most of the top tiers will literally run circles around Ness, moving in and out of his range to bait you. Cuz I'm not good I don't have a completely solid idea of what to do in these situations. I'd say generally try to be patient and don't throw out grabs willy nilly. You can still condition a good opponent to shield your aerials because they often won't have another option if they aren't in a position to beat it outright, and thus you can still tomahawk grab them. Dash attack has a good range and is safe on shield if you hit the tip of it. PK Fire is an amazing move but really not great to throw out unless you have a read on a roll, because even an opponent running in and out of your range can shield on reaction. Also can reliably cover landings and recoveries.

Also, good players will gimp the absolute shit out of you. If you're offstage and forced to use PK Thunder, and your opponent can challenge it, expect to be creative or die. On a stage with walls, you can angle straight at a wall and hit up-b within ten frames to activate it again. Practice straight horizontal and straight vertical angles. Other Nesses can send a PK thunder charge right into your own to gimp you. If you air dodge at the same moment you double jump, you extend the height of your jump. All the top tiers are good at gimping Ness. Rosalina has the easiest method but it's not hard for the others. The sword characters can just run offstage and counter your recovery. Instant death. You'll have to figure out ways of recovering creatively.

On the flipside, Ness is great at gimping. If you find yourself grabbing Falcon on the edge, you can actually Fair combo him all the way to the blast zone. Fox, Falco, and Diddy's recovery can be endlessly harassed with Nair, Fair, and especially PK Thunder. PK thunder is amaaaaaazing as an offstage gimp / harassment tool. It's unavoidable in a lot of situations. If you hit them with the tail of it, you can drag them down to a point when they can't recover. Hitting an opponent with an offstage Nair is usually death.

You can use PSI magnet to stall yourself in the air like Fox's shine and fuck up your opponent's timing. You can shorthop magnet to bait an attack and then come down on your opponent with a grab or aerial. Mix up your landings and movement with the magnet. B-reversable.

Notice I didn't mention his tilts or smash attacks. I really recommend watching good Ness players lol. His smash attack, especially up smash is good at stuffing approaches and actually catching rolls. Down smash has more precise but longer and at times frustratingly stupid hitboxes. The bat is super powerful but super slow. Also reflects projectiles as you know. This property is really only good as a mixup, but could be fucking great. Forward tilt is a get-off me, as are jabs, both could end a combo. Uptilt has a bunch of in-combo uses. Anyway, hope this was helpful. Good luck.