Pyukumuku for C/D. D is probably a better fit overall but when used to its "fullest potential", it can be worth a C.
At first glance you might think this is just some hot meme because I utilised it in a self-imposed challenge run that limited options to the point that it was worth using. However, I believe it truly has some efficient applications in Ultra (especially if caught in multiples, we'll get to that) that make it worth using in an efficient run. The most important thing to note with Pyukumuku is that you can't just use it like any old Pokémon and expect it to work. The inability to do direct damage outside of Z-Moves means it's not going to be incredibly useful against most trainers and as a result is reliant on Exp. Share to level well. However, the beauty of Pyukumuku is that it doesn't need many levels to function (actually, levelling too much for a point in the game can be a bad thing so level management is important to using Pyukumuku efficiently).
Its main function for most of the game comes in two forms: either it gets one shot and Innards Out deals some damage (This is NOT a bad thing), or it barely survives a physical attack and reflects the damage back with Counter. The former can do really well when paired with multiple Pyukumuku to either punch holes in a trainer's team or to cheese through a totem really quickly and only have to worry about the add (Three freshly caught Pyukumuku from Route 7 can cheese Totem Togedemaru [Need 150 HP aggregate to account for Sitrus Berry], punch a big hole in Guzma 1 [leave him with sub half health Golisopod], then boxed and brought for the Ghost Trial and cheese Totem Mimikyu, all without wasting items). The latter can be used to one shot certain enemies (For example, a mid-late 30's Pyukumuku for example can one round Totem Kommo-o by surviving a Thunder Punch and one shotting with Counter) and have health to spare for Innards Out. Even outside of Innards Out and Counter, Pyukumuku has some good supports including Helping Hand which makes the Aether Double fights with Hau/Gladion a lot more stable (especially Faba 2 as they tend to target Hau until he sends in Noivern, don't know if this is consistent between fights), Taunt can improve consistency in certain fights like Ultra Recon Squad member's Poipole, Safeguard can... block Thunder Wave trolls I guess. There are other moves that can help out (Memento could help a setup sweep in the endgame I guess?) but the moveset Pyukumuku gets when caught at Lv17-19 of Helping Hand/Taunt/Safeguard/Counter is good enough to last the whole game.
The biggest thing that counts against is one thing I already bring up: the fact that it is at its most efficient when used in multiples. This is because it's not indicative of how Pyukumuku performs by itself and even then it can still be used as somewhat efficient support, whether it be that you use it to punch a large hole in a Totem for something else to clean up or to manipulate the add called (e.g. Totem Ribombee's add depends on HP and if you drop it below half, it calls Blissey, which can be exploited by say, Roseli Berry Hawlucha to clean up the fight by destroying Blissey and then using Unburden Acrobatics to finish the totem*) or you can use items to resurrect and do it again to simulate multiple Pyukumuku (which isn't super efficient but good enough for Ultra Nec I guess). Whether or not you consider it as a bad thing really depends on if you want to go down the rabbit hole of whether a Pokémon should be considered by itself or are duplicates acceptable, but that's probably something worth discussing in that general tier list policy thread.
*: Not a great example after thinking about it a little more because of stuff like Helping Hand shenanigans from Blissey and Draining Kiss trolls, but you get the idea
It's not going to beat the whole game by itself, it's not going to route sweep things, and of course, it's something you are only going to use in certain fights. As team support however, Pyukumuku can be a deceptively efficient Pokémon when used correctly (and more efficient when used in multiples), and in Ultra SM, team support is a really valuable commodity (unlike most other Pokémon Games). Not to mention the number of fights you can beat/cheese really efficiently with it. For this, I feel C or D is an appropriate placement for Pyukumuku (if you want to just consider by itself, it's probably a lower end D at best. Good enough to stay out of E though).
Also shoutouts to the above post for giving me the inspiration to use Pyukumuku in my challenge run and allowing me to assess it much deeper.
EDIT: sb879 below also has a good point for other functions for solo Pyukumuku. Point is, Pyukumuku is no joke.
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Also on another note I kinda have to agree with D for Absol though if someone makes a really good case for C, I don't mind. Super Luck + Swords Dance + Slashes can turn it into a devestating sweeper that can really help for certain sections of the game, but the poor bulk and speed hurts, especially when pretty much every major battle features EV'd opponents. Not going to go too much into detail here.