A problem fox has against DDD, most of his kills are vertical. DDD is probably the hardest character in the game to kill verticaly, so he's going to be living forever.
Around 125%, a DIed, Airdodged, non-diminished uncharged usmash kills DDD. Without DI and airdodge, it's around 110% depending on stage... honestly, that's not too bad....and that's just from usmash. Bair is ridiculously easy to use in this match...and dsmash is amazing, too, so you can get horizontal kills there pretty quickly. Add to that the fact that Fox can edgeguard DDD quite well...and killing really isn't a terrible issue. Yes, Fox can edgeguard DDD. I know that DDD's aerials out prioritize Fox's, for the most part...but also, they're laggy and easy to bait out, so landing your own aerials isn't that hard. At the base of DDD's upB, you can shine spike him....under the stage if he's not careful... And at any point after the peak of his up B, or if he cancels it, you can hit him out of it. If the DDD peaks his upB, you can uair him out of it for a pretty low % kill. This is all situational, but in this match, Fox's problem is not killing DDD...
Fox's recovery is not the greatest, so you're going to have to make sure you do not get off that edge.
You're going to have to work a lot harder to get your kills than DEDEDE will. He hits you once near edge, there's a good chance you'll be done. Once you hit dedede, you also have to make it count by getting as many possible hits as you can.
Um, actually Fox has one of the best recoveries in the game...certainly better than average. Allyou have to do to not get killed in this match is be VERY careful with your second jump... if you do it near stage height and toward the stage, you'll eat a bair every time. so If you expect to jump toward the stage and illusion or firefox over, you'll die. But remember, DDD's aerial movement speed is quite slow.so usually, if you jump slightly
away from the stage, he won't be able to make it to you in time, and you can illusion past him... that brings me to the main point of the match...
The only way DDD can win this match is by punishing your mistakes. On paper, I would call the match around a 7-3 for Fox. Don't get me wrong...i think it's straight 5-5 though. Why? because every Fox (including myself) makes mistakes and tries to go for stuff they shouldn't....and just plays stupidly at times. DDD's lag time after most moves is slightly less than I always think, so I can't even tell you how many times he's shielded an usmash against me...
Fox has to be VERY aware of what he is doing and not fall into the same stuff over and over again. This includes, and is mainly centered around his recovery. When recovering, Fox has LOTS of options....even shine stalling for forever... so if you can keep mixing those options, DDD will have major trouble guarding you. The fact is that whether through edge guarding or just on stage Kills....DDD is able to kill Fox mad early with his moves... but for about 80% of them, he can't legitimately land his moves without guessing out something you'll do or punishing you for lagging on his shield.
Basically, keep mixing, and this match is really good.