How much do you think an experienced eye can tell about the quality of a fighter when you see him against bad opposition? How much can you tell from the "eye test" alone? And how much could you tell about a fighter's quality on film -- even somebody like Joe Louis or Roberto Duran -- if you didn't know who either fighter was? If you knew nothing about their records or backgrounds; just what was on film.
The eye test needs context. If a fighter looks bad but his opponent is known to have a style that makes fighters look bad, or if the fighter underperformed due to another reason (injury, bad weight cut, etc).