>>45598313the key in this context is arguably not the issue here for fauna, but you could say it is. a key is a set of notes, and normally songs stick to the notes in a given key because (mostly) any combination of those notes will sound good. going out of that key will more often than not produce an unpleasant or "out of place" sound unless you know what you're doing and do it properly (this is another topic entirely and it goes very deep so let's just leave it at that).
the issue here is that fauna is almost never following the melody correctly and you could say she's not even following a key at all, but rather just going up and down in pitch as she feels like. on top of that, she has trouble holding a note without starting to wander up or down in pitch, adding to the "out of place"ness. so when people say fauna is not on key, in this case they most likely mean she's not actually following the melody as it is in the original.
with all that said, GO FAUNA GO
I GOT TOOOO FAAAAR
TO LOOOOOSE IT AAAAAAALL
BUT IN THE EEEEEEEEEEND
IT DOESNT EVEN MAAAATTER