Hm...i haven't played a ness in forever...forgive me if my information i will post is outdated
PK Fire:
PK Fire is one of ness's staple moves, but it is very predictable and easy to shield. You shouldn't get hit by it often. When you do get hit by it, Tap + Smash DI up and you should be able to be high enough so he won't be able to grab you. The only other use is for edgeguarding. Ness will do PK Fire to dair, which is a ridiculously powerful spike. However, pikachu should never be hit like that since his recovery is so versatile.
PSI Magnet:
This is why pikachu's seem to think this matchup is actually difficult. Ness can absorb both thunder and T-jolt. However, pikachu beats him in the close range fight, so we should still have no trouble. At high %s you can shoot a T-jolt to bait the magnet, then run and u-smash him for the kill. Now since people momentum cancel, it will be MUCH easier to hit with thunder since he will still be in the attack. You have to limit your T-jolt and thunder usage, so you can't really camp ness as effectively as other characters. However, ness doesn't really have much to deal with anything you can do, so its still not difficult IMO.
Killing:
The main way ness will be killing you is with his B-throw. IF you are at the edge, this throw will kill you at 90%. From across the level, maybe 115%. So, when you are at high %s, try to go to the platforms. Ness's best killing aerial is his bair, and it is quite powerful. It will probably kill pikachu at about 125 give or take 10%. Pikachu can still kill ness quite easily with U-smash and nair. Since Ness can absorb thunder, u-tilt > thunder comboes are less reliable. This being said, if the ness is high in the air, you can bait a magnet and hit him with a nair or uair and get him off of the cliff, which leads to
Edgeguarding:
Ness's recovery is pretty bad. His up-b is really easy to gimp. If he is low and doesn't have his double jump, jump into him. If he up-Bs, try to intercept the actual thunder so he can't even get the boost. If worst comes to worst, go in his path and airdodge. If you are close enough, Ness's path will shorten even though you technically didn't get hit. If you get hit by PKT2, you are in a for a world of hurt. I have died from this attack at 30% (going across the entire stage, not just the short blast zone). This is ness's strongest attack by far.
Pikachu's recovery, adversely, is barely effected by ness. The only thing he can really do is send a PK Thunder out and try to hit you, but it won't disrupt you too much. Make sure you don't get in a pattern of recovering, because then he will hit with PK Fire to Dair, which will kill you at basically any %. You should be pretty safe off the edge against ness.
On the ground:
Pikachu has a better ground game than ness. Ness best spacing tool is his F-tilt. It has decent range, but our D-tilt has more range, so you can always outrange him. Ness will probably try to grab you at low %s and do D-throw > fair > DJ Fair (if you DI bad) for a good 20-25%. Pikachu cannot CG ness, with Forward or buffered down. So, the best thing you can do is F-throw > U-smash at low %s. Grabs will always be good to get ness of the cliff and edgeguard him. Pikachu's speed on the ground vastly beats Ness's. Pikachu has more range, and beats him in general.
Air:
I would say that Ness and Pikachu are even in the air. Pikachu has the speed advantage, but ness has more range with his disjointed fair, and godly bair. Ness's nair i believe comes out in 3-5 frames, 1 frame faster t0 1 frame slower than pikachu's nair, and is a good GTFO move. This means he will use it to try to break out of your comboes. However, with our overall speed, Ness should normally be on the defensive in the air. Uair comboes own ness pretty hard for a few reasons. When ness DJs, he dips down and it takes him a while to actually start moving up, which makes him unable to jump out of our comboes. However, he can maybe jump and AD really quick, which looks really weird btw. Pikachu can always follow him, but don't be too over aggressive, cuz ness can knock you away and do a bit of damage with his aerials and grabs.
Uh...thats about it...idk if i'm right or not since i haven't played a ness offline in months.