Regarding MK, IMO you can make this match-up really easy for DDD to succeed if you just focus on an extremly defensive approach.
DDD in general has a bad approach, so you might as well not approach in the match-up and rather punish every aerial MK pulls out, which simply just requires you to know the general ideas of MK's aerial game and think what he would do in a situation, so basically the shield to shield grab will wreck him.
Once you have the grab you have two options, the tech-chase(do it if you ur confident u can get a positive result out of it, or you want to get in ur opponent's head with good reads) or playing it conservative, which is what I do and am known for doing in norcal(simply pummel to bthrow every non-CG able character when they are over 35%, its generally racks up around 17% if stale)
IMO when the tornado comes out, as micheal hey has told me (Best IC player in WC, and most knowledgebale player in norcal and argueably WC) DDD realistically has no great anwser to it. Dtilt has range, but MK can hover over it to catch DDD's upper body, but a utilt spaced will get in, but IMO dont use it under KO% so it wont get stale. I just shield and move my shield with the tornado, which is effective with DDD's large sheild, but good MK's retreat when they see the nado isnt landing, so its difficult at times to punish it, unless they land right next to you on either side, cuz that a free tech chase or in my playstyle's case pummel to bthrow.
a utilt at 110-to high 90's% unstale is a guaranteed kill on neautrals, good fact to know since DDD lives to more than 140%(thats low, generally you should be living till 160%) every single stock in this match-up if you have good DI, remeber dsmash at 120% will put out a kill if unstale(good follow-up to a tech chase at high % is dthrow to dsmash out of shield, takes out their tech/backward roll and get-up options, unless they want to lose their stock)
I have beat ranked players in my reigion(norcal, son) who used MK on me, and the knowledge i have obtained from micheal hey and norcals finest really proves that its actually a winnable match for DDD,
always be a 60-40 MK IMO, noob DDD's will get wrecked, if you just punish in ths match up its a incredibly entertaining and IMO the weight difference will display itself as an incredible benefit for DDD, thats how i win my matches, haha