Shut down in the fact that Marth's (disjointed) range gives Kirby a lot of trouble, walling him out basically. If Kirby gets in, yes, he can do some damage, but Marth can, and will keep Kirby at bay with his range, and there's not a whole lot he can do to get around that safely. Marth makes a poor choice and Kirby gets in? That's when things can get a little more annoying.
Marth landing against Kirby isn't as hard as it seems, especially when you consider Marth has a better air speed and fast fall speed than Kirby. B reverse Shield Breaker is a thing too, something that I see Mr E do a fair bit more these days actually...
In the same vein, Kirby doesn't have it 100% free landing against Marth either. Multiple jumps are nice, but Marth outpaces Kirby in all areas (ground, air etc) and can follow his movement pretty easily (in fact, just walking work fine). Kirby doesn't like having to use up all of his jumps and then have to deal with a huge arcing hitbox like Utilt or Uair.
Kirby's Dair is one to watch for, mind. Though the very end of Dolphin Slash can hit Kirby out of his Dair, Marth has to be careful of it.
But generally, Marth's neutral is only made better by the fact that Kirby has poor range and average mobility; it gives Marth almost everything he needs to control the flow of the match better than normal.
That said, Marth:Kirby is a MU you see quite rarely, so none of us have anything super notable to point towards.
That's fair.
Perhaps I don't consider Marth's range to be much of an issue because characters like Ike are notably harder to get in on. Marth has a great disjoint in his sword, yes, but it's not that difficult to get through that with proper shielding/powershielding.
While Marth does have better airspeed and fastfall speed, that generally assumes Kirby is looking to catch Marth by jumping after him in the air. Shieldbreaker is a good landing option, but a little... telegraphed?
And while Kirby doesn't have it free landing against Marth, he is one of the characters that can actually just go directly to the ledge if pressured. It's what heavies and other characters with poor mobility aim to do, but spamming jump to get the heck away from pressure is a totally viable option (and I implore anyone playing the Kirby/Rosa or Kirby/Cloud MUs to do this immediately - no need to fall back into another attack).
Kirby's dair is generally a poor landing option - IMO (though many good Kirbies still use it, for some reason). The feet aren't disjointed or anything. Ffnair is generally better because of it's faster startup, improved safety, and ability to combo into anything dair can (at lower percents) - but jumping away is usually better all in all.
Marth's neutral is generally better in the Kirby MU, for sure, but I don't believe that Marth's range alone shuts Kirby out (I do believe Kirby still loses, natch - but not by much). He's probably better at that against Luigi (because Luigi *needs* to powershield everything), tbh. But in exchange, he has a stronger combo game, so.
And you're definitely right about the rarity of the MU, but our two best representatives - Mr. E and MikeKirby - do live in the same region. So the MU happens at its highest level more often than you might think:
https://www.youtube.com/watch?v=2BDWRaSuaPo
EDIT: While we're talking about copying Marth - the following moves can be punished by shield drop inhale:
- [-19/-31] All 9 Versions of Dancing Blade are Punishable
- [-20] SHAC B-Air (tipper)
- [-22] Dash Attack (non tipper)
- [-22] SHAC B-Air (non tipper)
- [-25] F-Smash (non tipper)
- [-26] SHAC F-Air (tipper)
- [-28] SHAC F-Air (non tipper)
- [-29] U-Smash
- [-32] D-Smash (tipper)
- [-35] D-Smash (non tipper)
(Courtesy of Phan7om)
Also, many stray hits can confirm an inhale as well, including weak utilt and reverse uair. So for the most part, if he can hit you with a combo starter of some sort, he can probably inhale. Of course, Dolphin Slash makes this all weirder, so some Marth-specific testing is probably needed.
(And I personally love b-reverse inhale as a landing option. :D)