A few years ago, I was watching the Junos (for some reason) and they had a clip of Justin in the studio with Usher when he was much younger, just after Usher took him under his wing, and he actually sounded like a decent singer. I don't want to overstate the case, but I could at least hear why he got signed. I saw him after that on some reality show (it might have been Dancing With The Stars) where he was performing without lip synch or autotune, and he sounded full on dreadful.
I can't really say he is devoid of talent, because at one point in his life he did sound good (or at least competent). So this seems more of a case of either a) someone with talent who spent so long in the star making and studio machine that he didn't maintain the ability he once had, or (and this is the one I believe) someone who had basic vocal competence but was more concerned with being a star and didn't care enough about the musical side that they never kept up their initial ability.