Fox is definitely stupid.
Out of anyone out there, I think his claim for being third best in the game, customs off and with a bit more contention but still arguably customs on as well, is near unrivaled.
Like Up throw Up Air on Diddy turns him from "best character in the game but measurable/comparable" into "demon bane **** my life", Fox' jab has similar application and is plainly way too good and should NOT exist (and any idiot who argues otherwise will feel my wrath). If it wasn't for this single thing that gives him amazing stage position, amazing damage/combo set up and guaranteed kill set ups on a frame two move he wouldn't be doing nearly as well as he does in tournament as he does now. Who are Fox' worst match ups apparently? The two characters who don't get repetitions on: Sheik and Zero Suit Samus. Sheik just falls into the ground and can always jab first, ZSS jab 1 is always faster too. Fox can shield before either can attack him though.
Fox has the third best results out of any character by a noticeable margin. Although not a perfect source, my scouring through smashboard's rankings and tournaments generally have me finding at least one Fox existing in every region with very good results. If we took away Diddy/Sheik right now, Fox would be overlooking the rest of the cast in a domineering manner still, there are just way too many tournament wins (1st place) being taken by Fox. NAKAT, DEHF, Megafox, SK92 when Fow doesn't show up, ksev. You will not be able to name even 3 other people bar those using diddy and sheik with this much consistency in first placements as Fox.
What are his weaknesses? Light weight, under-par grab game [for a top tier], recovery.
On his recovery, it isn't easy to gimp. He still has side b into up b (which can get to the ledge from close to outside camera view) that cover excessive distance, it is similar to Diddy in a lot of ways but slightly less versatile. With customs, it's different; he really only seems to gain strengths and applications while having weaknesses reduced (Wolf Flash and either of the alternative up-bs).
His grab game is solid at early percent, getting him follow ups and momentum he can capitalize on. At higher percent he'll struggle against held shields because it is generally very safe. With custom lasers he can frame trap from throws a lot longer/get chip damage, and with wolf flash he can combo out of throws depending on DI. As I've said numerous times now, swap Falco's throws with Fox' and he would be no.1 in the game, Diddy would be our savior, dark knight and everything he does would amaze us.
His neutral game of keep away and lasers is still strong/relevant in a lot of match ups. His falco laser turns him into a menace that is difficult to approach and practically always forces the approach from the opponent.
We don't see enough Fox play and hence why people underrate him. SoCal/West Coast in general are poorly covered by streams (well, they're always there, but struggle having more than 50 viewers), Texas is in a similar/worse boat. But I behoove you to follow through with tournament data research. It's all there. Clear and notable dominance.
The only mobility spec this character isn't basically top three in is aerial mobility. Everything else is up there/top. Walk speed, dash to run, fast fall, pivots, so on and so forth.
His ftilt, nair, bair and fsmash are reliable to be safe-ish on shield. Jab and Up tilt will rarely let you down either.
He -definitely- has close to the best nair jab and up tilt in the game. His ftilt might be too, very low end lag plus stupidly easy to combo/link up from. But Sheik ftilt boyz.
Up tilt, forward tilt, nair and jab are all kill set up moves and reliable damage racking and combo moves as well. He definitely has the grounded mobility to have guaranteed punishments on techs, ftilt and nair are not "gimmicks" or reliant on the opponent messing up.
What actually are his poor match ups? By any inclination of design, I don't see how any truly would be. Fox has everything Sheik would dislike when you don't consider his jab not being abused against her; basically same/better frame data and mobility, with kill power firmly being on Fox' side; Sheik's reliability of gimping him being the only downside. Against Diddy beyond everything we all know, his mobility allows him to keep the game "honest" a lot better than most characters, with a lot of reliable bread and butter moves that are for the most part better than dash grabs, Diddy's fair isn't as effective against him due to his slighter height and his speed.
I don't believe any other character in the game can claim to have as good of a match up against both Diddy and Sheik as Fox does.
Rosalina is a match up I just have not seen with this character at all but I can envision why it could be hard for Fox.
Luigi? Sonic? Like... maybe? I think customs (based laser) allows a solid keep away game against Luigi and keeps Sonic honest in neutral. NAKAT seems to be winning against False' luigi (arguably second best in NA) and run even/perhaps ahead of 6WX in head to head.
With the up coming patch I truly hope they fix Fox's jab. It's definitely one of the most abusive things in the game and contrasts with all known design paradigms/systems to have these things not exist. With that changed, the likelihood of Fox maintaining his dominance will go down by a lot, I'm pretty sure but he'll still retain a lot of the things that I believe keep him in and around 5th place. Jab to reverse up tilt seems just as applicable (reverse up tilt being 4 frames) and will be giving Fox bairs/uairs at kill percent too, he'll just have to work harder at getting damage / won't have as much of a frustrating to deal with frame two tool that people have to respect.