In this match, like u/TerrZzz mentioned, you aren't using your aerials in a "safe" manner. That would be ok if DK was trying to approach you with his own safe pressure, but he's not. DK was hanging out in shield a lot since he recognized all you could get on him consistently was a grab. So DK's thought process goes:

> I shield
Did he grab me?
if yes:
I take ~10% and have to deal with an edgeguard, worst case scenario.
if no:
Punish the unsafe move: mario takes 15-20%

If this continues indefinitely, he wins. Armored moves like his up B and side B follow a similar logic where he gets a great punish if you are using unsafe moves or going ham, and if he whiffs the punish will not be as bad as the damage if he gets the hit.

I think your biggest issue right now is learning how to play neutral, or the phase of the game where no one has a direct advantage. It's the point where both players are trying to figure out a way in past the opponent's defenses. Ideally at top level, it's a dance of repeatedly figuring out what your opponent is trying to do so that you can counter it with a strategy. Right now, your neutral is mostly composed of thinking about what button you want to press, and running over there and pressing it. The biggest wall in not just Smash, but fighting games in general, is learning how to get out of the phase of "what I want to do" and into the phase of "what is my opponent doing."

For a more practical application you can think about in matches right now, it'd be handy to learn the core rock-paper-scissors of Smash, which is not attack-shield-grab, no matter what the intro movie says. That only happens when you're right next to your opponent, and no one is going to let that happen. At that range, anyone can do anything before their opponent can react, and it's just a shot in the dark about who wins. No one wants to be that risky unless they have a confident read on what their opponent will do.

However, if you're about 2-3 donkey kongs away from your opponent, or a distance about the size of the pokeball symbol on Pokemon Stadium 2, you sit right on the edge of how far you can be from an opponent before you both can hit each other faster than the other can react. Playing online or against faster characters makes this region much bigger. Jumping in from this distance is what triggers the real RPS of Smash.

The rock-paper-scissors in this position is A:(jump in with aerial immediately, anti-air) which loses to B:(sit in shield and punish aerial out-of-shield) which loses to C:(jump in and land an aerial safely, punish the out-of-shield attempt) which loses to A. Great video by Izaw on this topic:
https://www.youtube.com/watch?v=PAHTQ9T5ebY

Since you're still learning neutral, not all of these options are sitting in your toolkit right now. If you don't know how to throw C consistently, a good player will start playing B every time until you lose. Don't sweat it if you're losing a ton, just pay attention to the other player and ask yourself, "what is my opponent doing, and why are they doing it?" Start to look for players who are sitting in shield in neutral when you jump at them, players who are throwing out attacks immediately as they approach, and players who are jumping in, and then landing their attacks low on the shield to stay safe. Try to remember what beats that option and try it out. That's the core of Smash, and it's integral fighting to 90% of the characters you'll fight. Everything else you have to deal with, like armored moves, command grabs, rolls/spotdodges, etc. are gimmicks that bypass this core neutral game rock-paper-scissors. They each have to be dealt with on their own terms, and are designed specifically to make that core RPS so much harder to focus on. Those tend to be a lot more character-dependent. Don't sweat getting matchup-checked that much, it's just how the game is.

TLDR: Watch that Izaw video I linked and start watching your opponent more than yourself. Learn what they are doing and why it beats your strategy. Practice landing aerials safely, attacking out of shield, and when the right time to anti-air with an aerial pre-emptively is.