By any metric, Deron Williams had a terrible finals series. He couldn't make a shot (2-16 from the field, 1-9 from three for the series), couldn't guard anyone (122 Defensive Rating) and was a general liability every time he was on the court. When all was said and done, he averaged 1.0 points and 1.2 assists in 12 minutes per game for the series.
Stats aside, you know you're not helping your team when the opposing side is actively excited for you to get into the game. Williams' former Cavs teammate Richard Jefferson spoke about Williams' brief tenure in Cleveland on his 'Road Trippin' podcast.