Optimisation has its limits, yes. The difference is that Nintendo is satisfied with targeting 30fps for a lot of games, and not caring as much about framedrops as long as the core gameplay is solid and works relatively bug-free.

They spent 12 months optimising Tears of the Kingdom, and it still has areas where there are slowdowns. It was not unfixable, they just decided it was good enough.

Its hard to compare games directly as each has their own constraints and dependencies. BotW for example was also released on Wii U, and that was a limiting factor. I don't remember stuttering in Mario Kart 8 Deluxe, but they did make it run at 30fps when playing with 3-4 players, if you mean that?

I think a more egregious example would be Hyrule Warriors in co-op, but again this is Koei Tecmo and not in-housed developed so they didn't have access to the same resources and tricks that Nintendo has.

They could have spent 12 months getting those 10-20fps moments smoothened out, but it was probably not worth the investment as 99% of players don't care or don't even notice when a game slows down a bit.