In my experience, MK wins this MU pretty convincingly but you have to play safely. Fox can't just throw out hitboxes like Sheik and expect to get in on MK, everything he does has to be deliberate which can make him a little predictable so take your time and punish. When Fox gets momentum, he doesn't get much off it if we DI properly and land intelligently; 5 mid-air jumps really helps us. On the other hand, Fox has a terrible time regaining his footing if MK gains momentum as MK has an answer for about every option Fox has while landing or getting back to stage. To be honest, the most threatening tools Fox has to close the gap is his jab cancel and early Up-Smash KOs.
However, while on-stage isn't too bad for Fox, off-stage is where the MU really falls apart for Fox. Both Illusion and Fox Fire get beat by the disjointed back-side of MK's N-Air every single time. MK can hop in place with his back to the ledge and cover all of Fox's recovery options. The hopping lets MK quickly fast-fall backwards into N-Air off-stage if Fox goes to side-b the ledge, N-Air in place if Fox opts to Side-B on-stage, and Fox Fire is just a death sentence. The best advice I could give to Fox players is to save your jump and attempt to slide by MK while he's in cooldown or recovering from the first N-Air. Never recover low.
So how does MK play safe in this MU? Respect Fox's options in neutral. If you go into this MU and just throw out moves, a good Fox is going to punish you badly. More than anything, watch out for his Jab! Fox will ruthlessly abuse his jab cancel if they get the chance. Play safe and jump OoS if he's on you. Fox's Dash Attack is pretty solid and, while it's not as hard to punish as MK's, it comes out faster. Fox's fast fall speed makes his short hop aerials land a lot faster than other character's but that doesn't mean they end faster. Fox's aerials have landing lag, not much but it's enough to grab or D-Air OoS or even just Jump if you aren't comfortable D-Airing OoS. I personally opt for D-Air OoS just in case Fox crosses me up. D-Air OoS beats most of Fox's moves on shield and is generally a smart way to drop shield against Fox but don't get predictable with it. Fox's Up-Smash is real solid but he shouldn't be landing it unless we mess up. It's a punish option; if he just throws it out, he should get punished because it has a
lot of lag. His D-Air to Up-Smash is a potential combo tho so don't over-commit and shield often when Fox is close. Shield in general is difficult for Fox to deal with since he doesn't get much off his grab and MK can land pretty easily so the juggle opportunities aren't even there. Again, don't get predictable with it but holding shield works really well against Fox as a hungry Fox will try to read a shield drop or roll and you can punish that. A general rule of thumb is that you always want to DI away from Fox. Up-Smash, Throws and follow-ups, D-Air > Up-Smash, Jab Cancel, Up-Air, they're all DI'd by holding away or away+Up. F-Smash, D-Smash, & B-Air are the exceptions (DI up) but they're all more situation then the others.
There's a lot of nasty things MK can do to Fox. At 0%, Dash Attack to Nado is guaranteed for a quick 25+% that also pops Fox into the air long enough for you to chase. Fox can DI away from D-Throw better than any other character, so stop going for D-Throw > Shuttle Loop after about 50~%. F-Throw's a little better but don't get predictable with it. After 50 or 60%, if you get a grab just go for pummel damage into Up-Throw unless you can get him off-stage with F-Throw or B-Throw; off-stage is where MK wins this MU the most so you want Fox off-stage as often as possible. From 40% onward, D-Air forces Fox to tech, jump, or land in knock-down state, read him and punish appropriately. Dash Attack > Shuttle Loop does work as a KO option since Fox is a fast faller, just make sure he didn't DI the Dash Attack properly before you commit to the Shuttle Loop. From the ledge, Fox's get-up attack is really bad so you can D-Tilt and cover half his get up options and the D-Tilt will shove him back off-stage for a potential punish; even on the ledge MK's momentum against Fox doesn't end, I wasn't exaggerating how hard it is for Fox to regain his footing. Side-B is the most threatening ledge option he has so expect it. You can honestly predict the Side-B while you're D-Tilting and run off and N-Air him before the hitbox comes out and after he jumps so he has no jumps off-stage against MK.
But that said, it's very difficult to punish Fox's Side-B on reaction when he has options. If you look at my match against Ksev, I opted for the on-stage punish almost every time. Part of that's just stream jitters/me being ill but a bigger part of that is knowing it's unsafe to go for a N-Air off-stage if he opts to avoid the ledge and recover on-stage or high. MK will have to make predictions for the most part, you can N-Air on reaction to Illusion but placing yourself in the right spot to do so is prediction-based. There's kind of a sweet spot by hovering above the ledge but the high and jump options make me a little hesitant to commit to it.
IMO, this is a solid
60:40 
's favor. Fox really doesn't win any part of this MU but, aside from the off-stage game, he doesn't struggle
too bad. Regaining momentum is also very difficult for Fox but he doesn't give up his footing easily once he has it back. Like I said, you have to play safe or he will win; he's a very solid character.
Katakiri vs Ksev video for those that haven't seen it:
https://www.youtube.com/watch?v=kt70lVA9tpI
