I'd imagine the increase in Zard-X usage is partly due to the actual sets its answers are running. Toxapex is pretty much going more spdef heavy than it was in early SM, Lando is generally gonna be running an offensive set, which Zard can OHKO with SR damage and a tiny bit of luck, Fini basically doesn't exist, Hippowdon is deader than disco, all of which really favor Zard. Additionally, I'd argue that scarfers in general are getting a lot less common, and some of those that are ran don't even answer it amazingly, with Scarf Greninja doing 48% max, Infernape failing to OHKO, Lele, and other slow scarfers have been getting more popular, and just die. Obviously if you face Scarf Garchomp you're sorta screwed, but that mon's less common than it has been since basically the start of the gen.
It is legitimately probably the best its been this entire gen, as hazard control is sorta possible atm, as well as the 3 attacks set basically barreling through all of its common checks fairly easily, again, due to pex running more spdef heavy spreads, and lando being run almost fully offensively.
Note: Worded horribly, will retype in the morning
Not to mention that I wouldn't exactly call scarf Garchomp one of the "best" applications of Garchomp in the meta right now. It's good on paper but in a balance heavy meta where switching moves(and having respectable power) is so important, it kinda just falls flat.
It will be interesting to see how Mega Charizard X progresses through this meta, because the current trends are heavily resembling ORAS in terms of supporting X's viability. Funnily enough, I remember the Charizard Y + Duggy combo being hyped up incredibly last year around this same time, and once it kinda fell off then Zard X took its place.
I'm both excited and not excited to see CM + T-Wave Clefable return, mainly because Clefable is a mon that really benefits from its versatile options and, in this kind of meta, can use its fourth move to do whatever it needs to do. I do think that the general power creep, including both Z-moves and fatter stuff like Toxapex, will actively knock it down from being "the" mon, but we'll see what happens. Overall, this is a change that is very exciting for the meta, and I'm interested in seeing how it goes from here.