I'm not terribly experienced, but to me it seemed like you were letting him have all the stage control and didn't challenge him at all for it, instead just chilling at the edge with SHFFL'd Nairs until he came at you. He completely dictated the flow of the match and when/where/how exchanges happened. The few times you did challenge him, things generally went well (although you seemed inexperienced vs characters of that weight/DK in particular), but sometimes you just went full Falcon and just Fair'd straight past him and got smacked with a Bair/got grabbed (don't do that!
).
I think if you can aggress but still retain your full spacing things will go a lot better for you (you certainly have a great grasp of your tech skill and general mechanics already). Also just mixing up your approaches/approach timings would help a lot with tacking on the percent you need to finish a DK off. Many times you would try to approach with a Short Hop Fair after he missed an aerial but just come in too late, leading to him just WD'ing back and going for another aerial/grab. If you had waited like a second more he probably would've thrown out another aerial that you were actually prepared for and you could properly punish him, instead of just rushing to punish and missing it.
You seemed to quit on using your JC grabs on anywhere but the very beginning of stocks, which I think could've been quite useful in a lot of the situations since it takes less time to reach over a large horizontal distance than a SH Fair (assuming they're standing slightly out of Fair range), and maybe overused Nair where other options would've actually had a guaranteed hit (Nair only seemed to work if he ran into it while trying to hit you, and at least imo isn't that great of an option in Neutral compared to the rest of Marth's moveset).
Random Notes/Maybe a Summary: Dash dance more and abuse your spacing, get used to DK's odd weight so you don't Falcon at him then just get grabbed since he doesn't fly like Pichu and barely staggers (if at all). Don't be afraid you move when you SHFFL
, doing it in place doesn't really gain you much, especially vs. someone as space-conscious as that DK, and usually leaves you open to getting rekt immediately after. Grab more and just try to get some safe percent by being decently aggressive so you can actually start getting easy combos off of even a non tipped Fair. Also... you have really solid combos when your enemy's are above you, but you never really got him above you with UAirs/UThrows/UTilts, even though Battlefield's a pretty great stage for these sorts of combos. If you can get him up on the ledge above you on Yoshi's/Battlefield, you can get the wonder combo of UTilt -> UTilt -> USomething -> probably an aerial and probably get a stock or at least like 50+%.
I might have missed a few things (Easter is hectic
) but I hope this helps somewhat!