I call the defense I saw above average because I don't value total yards as a measurement of how effective a unit is. Therefore, from my point of view, the Bills defense would appear from the stats to be the 18th best defense in the league, due to points allowed.
Except that I feel the defense was hampered by an offense that went beyond being casually bad. If the offense would've gotten touchdowns instead of field goals once in awhile, the defense could have played from a position of strength rather than a position of desperation...and opposing offenses would've had to call their plays with more urgency.
This line of thought leads me to believe that, with an offense you at least call close to average in terms of getting points on the board, the defense would have moved up at least three spots, meaning they'd pass into the top half of the league, and become (by definition) above average.
You by no means have to agree with that assessment, and it seems like most here don't, but thats just the way I see things. I feel the defense was really hampered by the incompetence of the offense, and put into difficult positions.