Singles set this thing probably isn't anywhere close to being OU, but I'm sure whatever tier it ends up in it's going to be beloved as a landmine/wall. Personally, I appreciate that it's at least versatile. You could run a counter/pain split set to force out non-substitute sweeper. You can also run it as a stally-mon if you weren't particularly worried about taunt. But the real kicker is the support this cutie can bring to doubles:
Pyukumuku @ custap berry
Ability: Innards Out
EVs: 252 HP, 248 whatever defence you prefer, 8 speed (purely to outspeed other Pyukumukus with no speed investment and maybe the odd Munchlax if the opponent is feeling as crazy as you).
-Pain Split/Counter (again this is up to you but pain split is better for recovery and special attackers, counter is a much better KO chance if you've invested in special defence)
-Purify/Whatever screen you prefer (Probably take purify if you're going with counter just in case you need the recovery)
-Psyche Up
-Baton Pass
This set is by no means a very powerful set, but being high tier isn't everything and it'll be pretty fun when it works. I went with Innards Out but Unaware could work too. Send it in alongside a setup 'mon and get off your boost of choice (let's say white herb shell smash) followed by same turn Psyche Up (thank you 5 base speed). Next turn either your Pyukumuku is in good shape (great!) or it's almost dead (Awesome! Custap Berry lets you go first!) Baton pass your stats to another scary 'mon. Purify, should you choose to use it, can clear up any nasty burns or paralysis (though this is one of the few occasions where Pyukumukus speed works against it).
Alternatively this set is versatile enough that you can send it in alongside a toxic orb guts sweeper. The guts sweeper hits with a toxic boosted move, and then your immensely slow Pyukumuku uses Purify to cure the toxic (Alolan speed stats are the real MVP here) and restore its own health. End of turn toxic orb re-applies but deals no damage to your sweeper that turn. Because turn 1 toxic tick is so small you can even miss a turn of purify to set up screens or pain split every so often.
Naturally the set is countered by taunt, and Pyukumuku isn't invincible especially if it gets double focused so you need to make sure your sweeper is putting in work too. I like this 'mon. It feels like it really has its own identity and it'll add another splash of variety to whatever tier it ends up in.