I don't think the defense was quite as bad yesterday, they gave up 300 yards, the Giants scored 14 points directly off turnovers and hit 3 50+ yard FG's, which is very rare.
They weren't amazing by any means but even adjusting for the opponent I thought they were better than previous weeks, which isn't saying much of course.
The previous week they put up 295 yards against a decent Rams defense in a tight game, so I don't think the performance yesterday was abominable as it has been most of the year.