From what I know there are first party games on Switch that are 30fps.
like Zelda BotW
Very much so there are! Breath of the Wild was also in development for a long time before the Switch hardware was probably finalised, that game works on the Switch as well as it can given the scope of the title. But the stuff which was designed more with Switch in mind like Splatoon2, Arms, Mario Ody, MK8D, Yoshi Crafted World (even running on unreal engine) all target and for the most part are locked to 60fps then with the hits taken to either resolution, visuals or both in the case of Yoshi.
Breath of the wild also probably fluctuates far more from 30fps than any of the other listed games drops from 60fps, I think their priority in creating and releasing that game was to release the game, the frame-rate was serviceable (was fixed somewhat after launch in some regions) but definitely the main prio was the game to be delivered as it was designed.
You take one walk through Korok village and you will never see anywhere close to locked 30fps, that region is so crazy how built up it is and then so densely filled with grass, leaves, npcs and other items, it's almost like they wanted it to be somewhere you didn't run through quickly... I dunno how it got through testing and wasn't cut back a little bit, even if you go there yourself and just spin out some of the grass the performance comes back a bit, but to release it and leave it as it is was always shocking to me.
Fancy hearing me on an amateur podcast with friends gushing over one of my favourite games? https://youtu.be/1I7JfMMxhf8