Expect either it to be on every team, or for Meditite to be used alot less (or Expulso's set becomes popular). With virtually no weaknesses aside from Spritzee's Moonblast (and Snubbull's Play Rough, I guess.), it can wall a good majority of the meta. With Will-O-Wisp, STABs only resisted by Dark types, and Toxic Spikes in a meta where Defog Gligar isn't a thing, it should be very good. I would expect a set to look like this:
Yamask @ Eviolite
Ability: Mummy
Level: 5
Shiny: Yes
EVs: 156 Def / 76 SDef / 156 SAtk / 52 HP / 68 Spd
Bold Nature
IVs: 0 Atk
- Will-O-Wisp
- Shadow Ball/Hex
- Dark Pulse/HP Fighting/Calm Mind
- Toxic Spikes/Knock Off
Will-O-Wisp is a requirement, as it burns physical attackers that can actually hurt its 21/27 physical defense with Eviolite. Shadow Ball is reliable, but Hex takes advantage of W-o-W or Toxic Spikes. Dark Pulse is second STAB, but doesnt bring much extra coverage, while HP Fighting insta-kills Pawniard. Calm Mind makes it a very bulky sweeper, making it untouchable on the special side and can burn things that can break through its mentioned 21/27 physical defense with Eviolite. Toxic Spikes works well with Calm Mind and Hex, letting Yamask easily stall Normal types and continue its rampage (it can also burn Fletchling). Knock Off is Knock Off.
The EVs give Yamask 21/27/21 bulk with Eviolite. It also gives Yamask 14 SpAtt, 21 after a Calm Mind. The leftovers EVs raises Spe by 1 point, I'll take it.
Now, the only huge flaw Yamask has is lack of reliable recovery (Pain Split plz). Spritzee is also very common, and beats Yamask pretty hard.
EDIT: tbh, Berry Juice seems tempting...