A few thoughts for what they're worth; apologies for the size of this but I don't want to condense it too much.
Ness vs Corrin:
First things first, Ness needs to establish as much stage control as possible from the start. It's imperative he be able to move forward and backward. I feel people tend to overblow the pin. It does suck because Ness doesn't trade well with the main hit of the kick and it's hard to punish if it hits your shield. We got some options against it though. If they instapin and it whiffs close to Ness (or hits his shield at anything less than max range), you can punish with a fullhop nair and they will be forced to either jump (not great) or just take the hit. If Ness was slightly taller I don't think this would work because he would likely get kebab'd, but as it stands it does. Size matters! Additionally, the hitbox of the kick itself does not cover Corrin's head so it's totally possible to punish the kick as it starts or is coming over if you're miles away for some reason (it's hard though and requires quick thinking), and the weak hit at the end of the move is perfectly fine to trade with. Kick is not that fast so this is feasible if you have a bit of a head start going in the same direction - you don't need much and this is also basically the only way you'll be able to land a grab in neutral. If they go miles away and are unpunishable it's still not particularly bad, because it gives you much-needed stage control.
Other problem aspects include Corrin's nair, which basically rules out airdodging through her in disadvantage and pivot grabs in neutral. Makes dash grabs harder to get as well, can't really block a nair then drop shield and rush in. Grabbing Corrin in general is really hard, like most swordies, and requires conditioning or them mis-spacing poke on shield, Ness landing a uair or something.
If you're a shorthop-happy Ness you will get fair'd out of the sky a lot.
A good Forward Smash is nasty too for obvious reasons.
Edgeguarding Corrin is hard. Dragon Ascent hitboxes beat PKT and the move is so damn big that it's hard to tailwhip with PKT efficiently. Even if you do successfully use the tail, if she's close enough she can use another up b and make it back easily enough since, if you got the tailwhip the first time, your PKT is probably not in a good position to try and get it again. Down Smash can work but is hard to make it work for the same reason, and if you try it while she's on the ledge you'll just get sharked. PK Fire is the most effective imo but that's not perfect either. Best option is probably to punish the ledge getup.
Counter-edgeguarding Ness isn't really that solid with Corrin tbh. When it does work it induces the salt, can confirm, but it's not a particularly consistent option. Ness doesn't stop in place when he activates the counter and he can just sail past and grab the ledge and get i-frames before the Counter hit actually comes out! It's kinda like edgeguarding someone with PK Flash. I get the "what if" but you're better off using a different option - you've probably got the stock anyways.
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Ness vs platform camping
Not a huge amount to say that hasn't already been said. PKT is helpful on high platforms especially, since nothing else Ness has is really that good. People love to talk about "the noggin" but it's not
that big unfortunately, especially vertically, and it's also slow because double jumping is necessary if you're going to pressure BF/DL top platform with it. Leaves you vulnerable too. In short, telegraphed, not safe, doesn't do enough shield damage to be worth the risk imo.
Platform camping can be a pain but I don't think the strategy generally autowins vs Ness. If a Fox decides to platform camp that can be a nightmare, but who heard of a pure-campy Fox? Other characters I'm not too sure, it's never really happened to me with anyone except a Fox who sometimes decides he might try it, nor have I seen a lot of it in other matches.
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Ness vs Rosa
Bringing this up because I respectfully disagree with Bowserboy3's conclusions. Don't take it personally or anything, I just want to further the discussion because there's a lot that can, and needs, to be said on this one.
First off. Almost every character can gimp Ness really easily if they bothered to try/weren't scared of being hit so I don't know why Gravitational Pull makes this MU so infamous. As it happens the matchup does suck but its not his worst, it's not unwinnable and it's certainly not bottom 5 in the game - all of which I have seen claimed far too often but thankfully not in this thread. But yeah, by your logic regarding Ness' recovery, pretty much every character goes even at worst with Ness bar a select few? That doesn't sit right with me. I'm not implying he blows any characters apart but I like to think he has some advantageous matchups...
Obviously Rosa starts the game in some weird kind of advantage state, which needs to be figured out. Rosalina definitely has an important advantage here. When Ness does hit Luma though it's fairly easy to keep Luma neutralised. Neutral air in particular is really good at keeping them apart. Up air is amazing in neutral here if you can make it work. Trouble is, jumping in isn't really a viable option against Rosa with Luma on deck, and grabbing isn't too great with Luma around either. Often you will resort to using PK Fire and dash attack to deal with Luma which is obviously not great and is indicative of just how limited Ness is in this phase. You will take a chunk of damage when trying to get rid of Luma. You just have to suck it up and make it count when you get the key hits in on Rosa.
If you get Rosa into (bona fida) disadvantage with Luma gone then yeah, Ness really makes it count. Offstage she gets destroyed.
If Rosa gets you into disadvantage with or without Luma, Rosa also really makes it count. If she does have Luma then god help you.
https://www.youtube.com/watch?v=HrkJlMJkg40
This, I believe, is a really good example of the MU being played as it should, certainly on Ness' part anyway. For anyone who doesn't know the EU scene, yes Tatuman is a very strong and established player, not some random.
So yeah - even if Gravitational Pull didn't exist... I would disagree and still say this MU is a solid advantage for Rosa. Even with GP the MU is not irredeemable by any means. Once the majority of Ness mains finally accept that it's not as bad as they think, we may be making some real headway.
Thanks for reading, it is appreciated.
edit: tidied some formatting to make it a little easier to read
But yeah, PK Thunder imo is a very good projectile, not amazing but pretty good. 
