Ok, so, today I did really well in Exposure 10, although I didn't get as much recorded play as I wanted and no stream time, whatsoever. What is relevant to this forum post though is that I also played a TON of friendlies with the local D3 who always counter picks Meta Knight against Ness, who had previously been kicking my butt, and am starting to get a good grasp on the match up. I took your guys' advice and began experimenting from that starting point.
To start, most of the tone of the discussion so far has been about how free MK is for Ness until you get off stage. I don't think this is quite true, MK seems to me to have a kit that is highly synergistic, gives him great offensive / approach options and in the right hands is just a devastating experience for anyone on the receiving end.
Neutral game: Against a truly competent MK there are not a ton of options here. This state of the game is going to be all about speed, movement and mixup. MK's combination of high speed, small stature and multiple jumps makes catching him in a PKF difficult, to say the least. Not to mention one of the few characters capable of consistently punishing poorly spaced / timed PKF's. It IS one your important tools in the MU, but don't expect it to do as much work as you may be used to, especially if your opponent is knowledgeable of and respects Ness's options. The threat of PKF is just as important here as actually hitting with it, forcing your opponent into options and patterns you can intercept. Fair is your other big tool in the neutral, but it isn't free. I'm pretty sure Fair will lose to or trade with well timed and spaced aerials from MK. This is where your speed and tech skill come into play, baiting the MK into making mistakes and punishing with a move that starts your combo tree. Patience, along with skill, is key in the neutral and will be rewarded as Ness's proactive tools pale in comparison to MK's, but Ness's defensive and more importantly REACTIVE tools can win you the game.
Combo game: Once your patience has paid off into a confirmed hit, it is time to change gears and go full throttle hard in the paint. Punish MK like he left you and your mom when you were young and never paid any welfare or even called at Christmas. Know when your combos end, but don't let up one second before they do. Yeah downthrow leads into downthrow when they DI away, but if they're any good they'll figure that out. Hit that SH Bair when they DI behind, but don't let up! You don't want to have to win the neutral state 3 more times for your bair to finally kill. Sprint after that tech chase, waveland or dair that platform they landed on, or proceed to edge guarding if you've thrown / knocked them offstage. Don't give them an inch until it's truly neutral again. MK is a character that wants to be the one applying pressure, not the one receiving it. His tech roll is kinda poopy, and he's the right combination of hit stun, weight and fall speed to just carry him all over the map if you don't miss a read and if you know all your options. This is a match up where I like offensive magnet linking together combo pieces. Your objective should be to make your opponent afraid, and second guess himself so he'll be more likely to make mistakes next stock. But, for god's sake, don't pursue some jank continuation that will get you punished. If you're lucky he'll try and punish your combo as you realize it's over, and then you can start a new one. If he goes home and posts on the MK forum complaining about the MU you've done your job right.
Edgeguarding: This is a joke, there is no edge guarding MK. Seriously, I'm not being a baby, I don't even care because you don't need it. MK is so light, you basically want to blast line him from stage. My MK will often go high with his jumps, then teleport somewhere over the stage and then Dair to the ground and L-Cancel. If you waste your time trying to punish that, which you can't, then they will have achieved the neutral state they are desiring, or you will get hit and they will combo you. They have so many other options, that it's not even funny. IMO, your best bet is to just threaten an edge guard attempt, maybe force one of their options and be ready to try and pressure them or punish the landing. Often times, I won't even care about trying to get a real edge guard but just spam them with PKT1 which they can't clank with because of the nature of MK's sword. This is highly viable since you aren't even looking to get their percent that high before any big hit or back throw will kill them. You might even pop them back onto the stage or a platform in a stupid way and get a tech chase opportunity. If you make them feel like they are being harassed in their element, the sky, you will continue the psychological warfare and put them on tilt, and get those stocks.
Small side note, Ness likes to kill off the top, but MK does that better than us. Even though MK is light, he also is a Fast Faller, so it's a little harder to kill of the top than sides. Bair and back throw are premium kill options, unless they are high up already when you land that up air.
Recovering: Another reason not to do real edge guarding. MK wants you off stage so bad. Anyone who doesn't believe the "Ness's up B is free, don't challenge it" bologna will quickly realize how good a simple edged Down / Forward Smash is against PKT2. My advice is to mix up sweet spotting the ledge from above, looking like you're going for the ledge from below but actually going more straight up and trying to land on a platform above them, and just recovering way too high. Nothing is really that safe, this is just gonna be based on reading your opponent's habits and mixing up your options. If they jump off stage to try and get you, use Mag stall shenanigans to evade an attack and go above them, or FF under them and dip back up and get back on stage. They still might get you, but *shrugs* that's MK's thing. My REAL advice is to not get knocked off stage.
Stage Choice: As is usually the case with Ness, this is a lot of preference. First ban for me is usually gonna be Dreamland. MK just utilizes the large stage and far blast lines better than Ness, who usually has the better recovery (distance) than his opponent. After this, your main concern is gonna be getting to a stage with close blast lines. This makes his edge guards a little better, but your kill options WAY better / earlier. If you feel your tech chasing, especially onto platforms is very strong, aim for YS, but settle for BF or FoD. If you want an uninhibited Aerial PKF game, SV is probably the optimum balance between unobtrusive platform and close blast lines. FD is a bit big but possesses no platforms to get in the way, and PS2 is medium sized with medium amount of platform play. As for counter picks, GHZ is premium. Close sides, high top and a flat stage with a mostly irrelevant platform. WW is super tiny, and you may want to play there, but I think the plats are a little awkward for MK. Ban Priority: DL, SW, YI, PS1
Spread and extra stuff: In light of recent progress, I'm inclined to concede it could be as high as 45:55, but it may still be as low as 20:80, as the really good players around here don't main him, so I can't be sure. I, and many of the very skilled and well regarded pros that I've discussed him with, view MK as a very high tier character in Project M, and he has a lot of advantages over most other characters. Like Falco, he is a glass cannon, but for slightly different reasons. If you greatly outclass and out play your opponent, characters like MK will seem like a joke. But, I challenge you all to MM very skilled players who main or second MK. I'm not trying to brag, but I think that I've held my own pretty well for a newer player against some really big names. But, D1 was consistently 3 and 4 stocking me back in April with MK because he's so good and has a Ness main back home to play against. I might do better now, but I wouldn't be that surprised if I didn't. And, if your opponent thinks that running straight at you, attempting to grab and down throw, and then fishing for a random dimensional cape hit, I don't think that qualifies for discussion.
matchup. its hard as (fire)balls.