I'm afraid it's not as simple as trained=good, untrained=bad. There are many artists that have little to no training that are still very good. Also, good (as in aesthetically pleasing), is subjective to the artist, his/her style, and many other elements. My main case in this point is Tom Waits. Try to get him to sing "Ave Maria", and you might be disappointed. But the content of the song, his look, his style, the music behind, and so much more made his singing practically iconic.