Is playing Zelda Breath of the Wild on PC legal?

[Pemalite]

The legalities depend on what country you are in.

In general... Emulation itself is perfectly legal.
And if you own a WiiU copy and drop it into your PC's disc drive and run it that way, that is also legal in most western nations.

Some countries like my own legally allow you to make duplicates of the media you already own.

Now where things get tricky is if you own a Switch copy of the game... Sadly you cannot drop that into your Disk Drive and go from there due to the lack of a Switch-cart port on PC.
Now if you were to torrent the game... Then you are also uploading chunks of the game, spreading the piracy of that particular copy, which is illegal.