She is mediocre AT BEST!
Well, I think that is an opinion colored by other things besides her actual ability, because I've heard her sing things that debunk your critiques, but nonetheless, compare that with the performance at the end of the show by the writer of that song to see "mediocre".
That Paul Simon performance was everything I dislike about him and some artists in particular. His strength is and always was composition, and the ensemble performance of songs that had a rare and touching human element. His strength never was, and certainly is not now at 73, being a guy who could stand unaccompanied at a mike and capture the audience, ala Paul McCartney or Tony Bennett (who, by the way, is on fire for a guy who is 88). Though the song choice was very appropriate ("Still Crazy After All These Years") it was, in my view, an arrogant and vain performance, and not in a good way.