My friend is an awesome Marth (I think he is top Marth in gamebattles) and my most common match is DK vs Marth with him so I think I can add something to this discussion.
The biggest thing I notice in this match up when I use DK is that although its a very close game, Marth definitely has the edge in momentum. Only ftilt max range, dtilt max range, retreating bairs, giant punch, and retreating uairs (lol I know its silly but hey) are safe on block. Dair spike and footstools wreck DK's recovery. DK's shield is too small for DK and he gets shiled poked by ftilts, dtilts, and ac nairs when his shields aren't at max size. The constant barrage of safe approaches and mix ups Marth has is very frustrating and fair combos to uair juggles get ridiculous after a while. DK's size means many reverse fsmash tippers, both hits of dsmash hitting, dsmash hitting you with the first hit when your behind Marth, and other ridiculous moments. What keeps me going is landing that 9 punch/ mind gaming in a charged fsmash, taking him out at like 50%.
His Marth is always in the air so dealing with his sh approaches is the first thing I learned to do. If he gets sloppy (pretty rare) with his fairs shield grabs work well. An upwards angled ftilt is key for DK in this match up. It outreaches Marth's sh options and 12 damage is pretty good. His most annoying approach however for me is the ac nair. It shield stabs, it is pretty strong tippered when DK is 100+%, and ftilt does not work as well since the hitbox lasts awhile and ftilting it too late or early means getting hit. DK's dtilt works very well against this approach because it shrinks DK's hurtbox a lot and Marth's legs are pretty close to the ground. AC nairs hit high so dtilt works wonders here. Once DK has a fully charged giant punch sh approaches can be punched through with practice (especially fairs).
Although DK has trouble with jugglers and Marth is awesome at juggling, DK has a few tricks. When Marth goes for some uair/utilt/usmash juggling at higher percents DK's side b comes in handy. It stalls in the air and resets fast falls so using it wisely can throw off Marth's rhythm but doing it too close to Marth is punishable and pretty dumb. Knowing when to fast fall, airdodge, and double jump also help. To mix it up every now and then do a dair, nair, or fair depending on Marth's relative position and your height while playing around with the fast fall timings. It can be surprising how often they work. Charging the punch high up can bait Marth into thinking he gets free hits but DK can cancel it well before Marth gets to him. If it doesn't bait Marth into using his double jump to chase you get free charge. Another thing to keep in mind is that during DK's up b he has better airspeed than usual so using it to retreat to an edge or a lagless landing on a platform is an option.
Marth is good at juggling but he is also a little weak to juggling. Being below Marth is an awesome position to be in and DK's cargo uthrow is an awesome way to put him there. A jumping cargo uthrow to uair is one of the best KO's you will ever get (KO's before usmash) and Marth will have a hard time stopping it. He can airdodge but then you can do another uair before he finishes the airdodge. If the Marth dairs it is usually too late. If you mindgame in a baited airdodge you could fair or giant punch depending on how much higher Marth is than DK. DK's utilt juggles do not work that well but it still leaves Marth in a bad position where he either airdodges the next one and punishes DK or Marth just got baited into airdodging and gets punished with something else like a dsmash.
Gimping DK is not that easy if the player knows how to control his up b and that sometimes the way forward is really backing up a little bit. My friend is a very aggressive edge guarder so getting back to the stage is always a tough fight. The first trick I have learned is that fair really works. It works because it is disjointed (DK has arms of steel). It makes sure Marth doesn't get too close if you space it correctly. A lot of times that means backing up. Marth does not have a great horizontal recovery so he cannot afford to chase you too far and fairs give you some breathing room (or Marth gets hit). Another trick to not getting gimped is mastering D's up b. Most DK's I have seen look like they do not know you can go backwards/ slow down DK's up b. This is mostly for when DK is high enough to get on the stage without the ledge because otherwise they could be smart and just edge hog DK if they see this coming. It makes easy edgeguards like Marth's edge hopped dair ineffective.
Gimping Marth is pretty hard but still doable. Cargo stage spike can work but if DK is too slow or Marth doesn't have enough damage he can footstool DK to death (my friend is a mashing machine though so this happened a lot). Cargo dthrow is an excellent set up for edge guards. If Marth used up his double jump before being grabbed and didn't land its an easy gimp since his up b has no horizontal range and he doesn't get his double jump back. Weak bairs and weak nairs (the nonsweetspot that does 8 damage for both) can gimp from a cargo dthrow at low damage since he can't really DI up from really weak hits. DK's up edgestall is very good makes edge hogging Marth a little easier. Bair edge guards do ok for DK too but most of the time Marth will try to recover low so it is only good when they try to recover from high up. Marth survives spikes pretty well but after like 50% he should die if you land one.
Dancing blade used to bug me a lot but now that I know how to deal with it, it's not too bad. When blocked DK can shield grab after the third hit. A nasty trick I learned is DI'ing up during the multi hit 4th db and side b to punish it. It works and he never does that anymore lol. Shield breaker renders DK's shield useless if DK blocks it. He mostly uses it as an edge hopped poke (pretty effective to boot) but if Marth ever tries to mindgame in a charged shieldbreaker or just delay it thinking DK will spotdodge I just stay barely out of range and charge an fsmash which really hurts. Up b out of shield is annoying but when DK has 50% or less he can punish an up b that hits him with DI and fast falling. Counter is all about mindgames so nothing to really say about it besides charge a smash if Marth does it on reaction to your smash attacks.
In general I would say it is pretty even with a slight advantage to Marth but not enough to be a counter pick. Marth is better at approaching safely and tries to overwhelm DK, but DK has better reach, power, and outlasts him. DK has to be more cautious and outplay Marth to win. 55-45 Marth-DK
But this is too risky i guess and i will just stick with going for a dair on DK since his verical recovery potential fails.
