Was reading through this thread and felt like throwing my hat in the ring since I've not posted here in a while!
Although Mtar has obviously seen better days in natdex and just in general, I still hesitate to call it outright bad and say its niche flat out isn't at all important in the tier right now. Mtar is able to pressure Flying-types like Zap, Molt and the albeit slightly of the boil Yard, well, while also threatening Stealth Rock and most importantly Knock Off and Pursuit allowing it to threaten staples such as Gholdengo, Gking and Mtios. While not an answer to any of these bar Gking by any means, it definitely has positive traits into them offensively. When paired with teammates such as Zapdos, Corviknight, Iron Treads and Gking in teams
like these it can be utilised to force pressure on defensive cores with its combination of Stealth Rock and Knock Off, as well as trapping important pieces via Pursuit. Although its showings in NDCL have been pretty paltry, winning only 3 of 7 appearances in the first 4 weeks, managing to get used 7 times surely means theres something worth using there.
To the rest of what slopcat said, I pretty much agree, Kart is fundamentally outclassed at nearly everything it tries to do, and even when it does hold some form of niche it's extremely limited. Same applies for Water Absorb Clodsire which pops up on random ladder teams which is just an attempt at trying to compress loads of roles into 1 subpar mon to try and compensate for bad building.
On the flipside I do also agree that stuff like Hatt and Offtran can be looked into more than they are currently, especially Hatt.
CM Hatt seems really nice right now, even despite Gholdengos huge presence, being able to answer things and apply pressure to mons like Zama and the plethora of Dragons in the tier. Even without turns to setup Calm Mind, Hatt can apply great pressure through Psychic Noise and Mystical Fire, which a large portion of the tier struggles to switch in against. Not only this but Hatt also has great variety when it comes to being slotted. Support sets can be very helpful support on some Offense builds, notably Weather Builds such as Sun, where it is used as a way to deny hazards, gain momentum and cripple offensive threats with Nuzzle. The more widely spread Calm Mind sets can fit as a wincon on many structures, BO, Offense, Screens HO, you name it, putting in work as a wincon on most. My HO pilled self has loved to use it on Zama screens, where between Leftovers, Draining Kiss and Calm Mind it can easily find setup opportunities and break through common defensive cores using Zap, Glic, Pex, Ferro and the like.
Offtran also seems to be useful in a Ghold pilled metagame, with Grass Z being the main option giving it a way to nuke Mola and Bro, although sets using HP Ice and potentially even Taunt seem nice to nuke threats such as Gliscor, Lando T and prevent pivots such as Gking from pivoting out and Pex from recovering. Although extremely situational, Rock-type moves such as Power Gem can also be used for foes such as Yard, Molt and Volc, although dropping one of the more traditional slots like EP probably isn't worth it most of the time.
Closing thoughts are that I really like the spot the metas in right now and am having a lot of fun playing and building it, sorry if any of this came across as ramble I was kinda js letting my conscience stream out lol