I see your point guest, and you are right - I would not give a message like that. The example you gave may be tactless, but I would also say that it isn't very constructive criticism. I might tell someone that the sentiments in their song were sloppy or cliche, and the tune really didn't attract my attention, and the vocal range was a bit limited. I would then offer some suggestions that would make the song more interesting to me.

I do disagree with your suggestion that "it is a bit strong for someone who is neither a good songwriter nor performer to denigrate a tryer when they themselves could not do half as well."   I can't hit a curve ball as good as Mike Piazza, but I know enough about the game to complain when he isn't making an effort. I can't act, but I do know when I see an actor walking through a part in a film.   I may not be able to play guitar, but I know when the instrument is out of tune. I might not be able to write a poem, but I know when one touche me.

Everyone is entitled to an opinion, and the artist should always consider the reaction and the source. Music is owned by all of us, not just the writer of the song.