"Casual Party" games in general aren't doing that great on Switch.

I mean 1,2 Switch has done ok mainly because it's the only other Nintendo published game at launch that had any marketing but it's sales are still likely going to be lower than things like Wii Music and Wii Party U, which are considered duds. It's also dropped off the eShop charts like a rock getting outsold by things like Neo Geo games already. In Japan it looks like it's run out of gas since Mario Kart 8 came out and gave people something else to buy. 

The JackPack Party Pack doesn't seem to have done that well either, I think it peaked at like no.4-5 before falling way off the eShop charts.

Just Dance has been a giant dud, the only Switch game to sell less at retail is the even "floppier" Skylanders. It's being outsold by the Binding of Issac physical and is nowhere on the eShop charts.

Wii+Wii Sports was £180 in the UK

Switch+12S is £320

12S is no Wii Sports so doing the numbers it's doing given the price/constrained supply is pretty impressive, imo. Whether you like the game or not is insignificant, it's about the jobs each game does for different needs and demographics. When the greatest videogame maker in the world devotes it's resources to new types/styles of gameplay you really should pay attention instead of lumping mini-game compilations, fitness, motion control, party and accessible games into the same 'muh cazualz' basket (pssst Just Dance isn't a minigame collection).

Besides, shouldn't you be playing Arms on your iPhone ;)