The more motivated you are to vote, the more likely you are to vote it a five. A mediocre video is characterized by a lack of votes, not a preponderance of three-star votes.

Back when Youtube rounded the average vote result down to the nearest star, I joked that Youtube videos really only had three ratings:

Five star: New video (eventually it will accrue a vote of other than 'five' and drop to four stars) Four star: Top notch (mainly fives, a few troll one-star votes) Three star: Disgusting trash (at least many one-votes as fives)

You don't see anything less than three-star since it will have been deleted by the time you get there.