Objectively speaking, it'd come down to it being between Marth, R.O.B, D.K, and maybe G&W are his even/hard MU's. And that's because these characters have a punish, or 0-to-death game on him. But, at the same time, Fox still has the same tools, but with different properties, so he can still hold an even MU spread across the board. It'd say the real counter-pick comes in from the combination of character and stage. I wouldn't say M2 completely because he can run around and away from him really quickly. Just don't stand still, avoid bad shield pressure that gets you grabbed, and learn a bit more of a way to get more from your neutral. Fox is an all-around character that has something for every situation. And all of his tools, while some nerfed, can definitely handle the situation. It takes a lot more thought in these MU's because a grab=death or off stage, which is now a lot less survivable for Fox.
What's the Fox vs. Zelda matchup look like in the current version of PM?
Also, I remember being told that Fox vs. DDD matchup, is in DDD's favor. Is that still true?
I'm inclined to disagree, and I play D3 and Fox. I was able to run sets as both characters in both aspects of the MU, and I can tell you that D3 at best gets his placing in the MU as a 45-55 losing. This is due, primarily, to his size and speed of his moves. Granted, some moves are a lot faster than some, he lacks any way to break pressure when fox is all over him. Its all reliant on either a roll, a grab out of shield, or something even slower and unsafe like Up-B OoS, or nair OoS. D3 also is subject to being too big to completely evade laser fire.
All of the aspects that D3 wins are based on chain grabs, tech chasing, and killing off stage. His stage game is relatively based on getting stray hits that can lead to these things via minions, W-dashing, jab (1 or 2 hits) OoS grabbing/jump cancel grabbing, etc. Either way, the majority of times I see D3 win the MU is because the Fox that fights him is severely unversed in the MU, doesn't have the tech skill necessary to punish bad waddle dashes, can't pressure him, and also cannot impose any form of offense at all. Also, after like 20 percent or so, D3 can be subject to u-throw u-air combos, even with good DI. nair is too slow, and Dair requires a position that the fox probably wont be in in order to break out of the combo. Which I believe kills him off the top of battlefield at like 115-120. possibly less I think at PS2.
Lastly, from my experience from grinding the MU at Final Boss this recent weekend in San Jose, I can say that as long as I committed to a camp, the D3 couldn't stop me. He could try to W-dash to cover distance, but then I would just run in, and punish the recovery by jumping over the tossed minion, and nair/drill shining to an u-smash or grab to u-air. However, D3 has the benefit of mixing up his recovery with the large hitboxes he can swing on his way back with full resources. He can also still threaten space below him with his up-b. However, if the fox is just going to commit to defense/punish game, you can recover all day, but the opening that got you in that position to start with will only keep happening until the fox commits to a kill option after racking up damage on stray hits, and wave shine punish combos too. That's all on recovery I could really gather from the time I had. Sorry I can't remember the D3 who I played, but I was told there was another one there besides me that was actually from the Bay area. I assume that was him.