I don't think the frame rate is a hardware issue but can be attributed to poor optimisation. A lot of more demanding titles run at a higher and/or more stable frame rate on the Switch. Of course, the game would run much better on more powerful hardware, but that's hardly an excuse. Animal Crossing doesn't simulate physics for characters, for instance, and doesn't have to render too many objects at once since the camera can't be rotated freely.
Another indicator of subpar optimisation are the loading times. There is no reason for the game to take this long to load. As a result, I think that optimisation just wasn't a focus during development, which also makes sense: the game is no fast-paced action title that demands 60 fps at all times to be playable. Moreover, the frame drops are only ever noticeable if you spend a lot of time decorating your island. Most players will probably never experience them.
Another indicator of subpar optimisation are the loading times. There is no reason for the game to take this long to load. As a result, I think that optimisation just wasn't a focus during development, which also makes sense: the game is no fast-paced action title that demands 60 fps at all times to be playable. Moreover, the frame drops are only ever noticeable if you spend a lot of time decorating your island. Most players will probably never experience them.