I have been writing songs on and off since my teenage years, and still waiting for the "big breakthrough". Oddly enough, one of my stupidest songs is the one that gets the most requests to sing it again: not a great song in anyone's book, but makes people laugh. Of singer songwriters that I know of, I don't think you can make such sweeping generalisations as " good" and " bad" songwriters. Even the world-famous earn-my-living-doing-it ones will have stronger and weaker songs ( in my opinion - someone else will equally think that the song I rate highly is rubbish, and vice versa ). And I can think of one or 2 writers who are highly esteemed by many, but I just don't get why they are thought to be so bloody marvellous. So all in all it's pretty subjective who's good and who's bad, or which songs are fantastic and which are crap.
Johnny J has mentioned the the Edinburgh Folk Club Songwriting Competition, where there are 2 systems of voting: 3 appointed judges who decide 1st, 2nd and 3rd places, and then the audience vote, where each member of the audience can vote for what they thought was the best song. The results from the 2 systems are usually widely at variance with each other: only on one occasion in many years of running the competition did the judges' results tally with the audience votes. (Of course, the audience vote could be swung by those who bring their personal fan clubs in - must start sending out a few emails!)