And last year it was Fire Emblem, Link's Awakening, and Pokemon -- all which had frame dips and slowdown out the ass. Pokemon was especially egregious. If a ton of first party Sony, Xbox, or ANY publisher was consistently releasing titles in their big IPs with these issues they'd be getting flamed for it. Ubisoft already does.
AoC and XBCDE is more than just "kinks," too. Hell, ACNH isn't a looker by any stretch and still has significant FPS drops on a busy island.
And all of them weren't made directly by Nintendo. Also for three games you had Mario Maker 2, Astral Chain, LM3, and DQ11 which Nintendo did publish.
The point is, most of Nintendo's games run fine. It isn't like Nintendo's game quality took a dip.
Have you played ACNH? The game is a looker, it just isn't as apparent as other game and the frame dips only happens after you deck your island or are online, which is common in almost all games that gives you created control.