>>713117812It's a conscious shift towards the cartoony. It's why you got the redesign in Wonder, why Mario is stubbier in MK World, why DK is now so animated. It's a very clean break away from what was for the most part still evolutions from 64 and SNES-era designs.
I certainly wouldn't say it's for the worse but it's pretty clear across a lot of their Mario-adjacent IP (Warioware and DK particularly.)
ARMS was just rough because it wanted clear designs for the sake of readability, the fact we've had Splatoon 2/3 since then with some great designs definitely suggests your timeframe is off-base.