You can't just say "it should be easy to get it to 60", if it was, they would have done it. This is Nintendo EPD, one of the best development studios in gaming, Obviously the visual fidelity was prioritised in the overworld. They prioritise (and hit) a locked 60fps in games that truly need it, Mario Kart, Splatoon, Smash etc. It might be 2D gameplay but it's still a 3D game.
I don't think it indicates anything of the sort. Animal Crossing looks lovely, Link's Awakening looks BEAUTIFUL, you said it yourself, Luigi's Mansion 3 looks fantastic. It's not a quetion of quality slipping, they prioritised visuals in a game that is in essence a VISUAL remake. You're playing it for the visuals. Would I like to see 60, absolutely, that would be great, but it obviously isn't possible in its current state.
Maybe they'll improve the game. Unlike other platforms, Nintendo games tend to look and run better the closer you get to release, so who knows.