A2 I wish we could have played at that one Brooklyn tournament that you came too xD I probably have the most matchup experience vs Mario in the entire world.
Alright, let's do this. Cable was right in that if Mario keeps up the pressure (especially if DK lacks a double jump), DK is basically screwed. Mario's combo potential vs DK in particular is nuts since DK has that blind spot diagonally underneath him while he is in the air, which is thoroughly punished by Mario's up airs/up tilts at low percents. Stay on DK and space fireballs when out of his range altogether to enable grab/b-air/d-air approach mix ups, and it'll be really tough for DK to shake you off.
However, DK's spacing game is ridiculous. F-tilt is sooooo good vs Mario lol. D-tilt is very safe as well if spaced. Both clash with fireballs and DK can react to use any of his moves to clash with fireballs so that they barely hit, if ever. I prefer using single jabs to cancel them, walking around to space myself so that the jab connects with the fireball, allowing for any move afterward as a follow up should Mario choose to approach.
Retreating fastfall b-air > Mario. DK can literally do that all day and Mario will never get in. For the sake of extreme theorycrafting, your only choice is to like run in and power shield it or maybe FLUDD as a mixup which, which btw you are definitely underrating since it is very useful in disturbing our spacing/wall game. FLUDD usually causes frustration/panic which is probably it's best feature imo xD Not to mention if you FLUDD a DK while he is falling with a punch, he is cape fodder, and possibly dead.
Gimping is a VERY real threat for DK in this matchup. Can't express that enough, no matter how smart the DK is, his up-b is very predictable, and almost always has to travel horizontally. Simply read when DK is going to up-b if they are trying to sweet spot to the edge from an unsafe distance, and cape it... lol. Ask Kirinblaze, it's basically his job.
The only real way for DK to avoid this is to recover by not using up-b via reverse charge canceling with b-airs or by recovering very, very low so that the double jump occurs parallel with the edge, and the up-b sweetspot occurs vertically rather than horizontally. In this case, I recall being caped, yet since I was not moving in a horizontal direction, I was still able to grab the edge and recover (very rare case and cape should just be avoided at all costs lol).
In addition, FLUDD causes DK's up-b to stop in mid air for a few moments, in which case you can choose your best method of punishing, if he is coming in too high to sweet spot- cape. Otherwise? You can probably grab the edge or simply shield to set up a punish if you see that DK will not lagless land.
I'm merely grazing the surface of this very entertaining matchup, if you guys have any specific questions, feel free bc I know this matchup like the back of my hand. (pimp smackkkk)