Regardless of whether you can think she can sing or not, surely you're smart enough to understand the distinction between singing and judging (otherwise I don't know how you could be smart enough to find your way to on the internet :D). Surely you can see the bigger picture; the judges aren't personally judging them according to their own singing abilities. By approving of a singer she's not saying she's "you're as good as singing as me or better", just as she is not saying "you're worse than me" if she rejects. Much like the other judges, they're RELYING on their EXPERIENCE in the music industry to contribute a sense of what they feel (based on what they've seen and experienced) is appropriate to "make it". Otherwise wouldn't the producers of American Idol make a bit more of an obvious effort to pick 3 judges who are of the best voices in the country??