i suppose that, by your definition rhett, tom waits would also be a "bad" singer. maybe you should enlighten us as to who is a "good" singer in your book. are you going for perfect pitch (e.g., an art garfunkel or thom yorke) or do you go for a vocalist whose voice is suited to the style of music he/she is singing? if it's the latter, then one could argue that no one is a bad singer.
i saw melt banana live once and their female vocalist ranks up there with yoko ono as one of the screechiest, most obnoxious vocalists IMHO...there are people in the world that enjoy it though. just because i don't care for it personally, doesn't mean she's a bad singer.