People have mentioned movement stuff which is a solid thing to work on. In addition to that I'd say work on approaches, particularly spacing aerials. There were a few times throughout this set that you approached with a Bair, missed because the hitbox is mostly behind you, and landed right in front of him for a free punish.
Another thing I'd say to work on is recovering after being comboed. I saw a few rogue get-up attacks that cost you some more damage.
Off-stage as well. There's not a whole lot you can do considering Marth's strong edgeguard game, but I did see a moment that I wanted to point out.
Where you are at this moment requires an awkward angle to sweetspot the ledge, and the Marth recognized this so he wavedashed back to setup for:
This moment right here, which ended the first game. There wasn't a lot of time to react to his wavedash back, but in certain cases like this I think you would have been better off going for a much higher angle, almost like: \
Which would have put you out of range for the f-smash/d-tilt, jab, or counter.
Totally forgot to mention: What are you doing counter picking Marth's to FD man???
Askin' for trouble
I don't think Fox necessarily has any "bad" matchups, but Marth on FD is a strong contender for the title