To me, sentimentality is, with very rare exceptions, a killer for songs. I can't stand the cheese in Paul Simon songs. I would definitely say that The Boxer is showy, with all of the novelty instruments and techniques (particularly the elevator-shaft drum echo). Compare the BOTW version to Dylan's version; ok, so the "duet" might suck, but the arrangement is less melodramatic, less flashy. I don't find Simon's melodies particularly beautiful or his lyrics particularly moving or well constructed, but to each his own.
Yes, every act has influenced bad music (a fact I admitted), but I think that when a group's influence has such a wide-ranging negative effect, it can be considered a "crime against music". (You're taking this far too seriously, I imagine. If someone said that the Sex Pistols committed a crime against music by making tunelessness a virtue, I'd laugh it off and figure that it's only their opinion).
I happen to think that The Dangling Conversation is one of the absolute worst lyrics ever written. Their other songs aren't far behind (and I only cited one line because it was the one that first came to mind; I could post every lyric Simon's written if you like). Again, it's a value judgment. To some people, Jim Morrison is a brilliant poet. To others, he's a hack. Neither position is right or wrong.