A somewhat poor, though adequate, performance of a Bach fugue with an instrument that doesn't have great tone will still convey a great deal of Bach's meaning.
A performance by average vocalists of a Simon and Garfunkle tune, backed by some average lounge band will just be a laughable farce.
It can't be saved by the musical content, because that is there in insufficient quantity.
Pop music is all in the nuance and tone that doesn't get notated in sheet music.
The result is that performing a convincing cover of some pop music requires a very high musical standard. Whereas a five-year-old kid can play some passable Bach on the piano that we can enjoy as Bach, just by hitting most of the right notes from the sheet music at mostly the right time.