In the past 15 years (as far back as can be readily searched on espn):
In Iowa's 5 most successful seasons, they've had top 20 defenses in both total yards and points allowed in every year, and in those same seasons, the best offense was in 2008, where they were 53rd in total yards/game and 33rd in points/game. On average in those 5 years, their defense was 12th in both yards/game allowed and point/game allowed, while their offense average was 81st in yards/game and 57th in points/game
In Iowa's 5 least successful seasons, they have had no season where their defense was both in the top 20 in total yards and points allowed. In 2007 they were 13th in points allowed, but 39th in yards/game allowed. In 2014, they were 19th in yards/game allowed but 41st in points allowed. In those 5 years, the offense was never in the top 20 in yards gained or points/game. On average in those 5 years, their defense was 47th in yards/game allowed and 35th in points/game allowed, while their offense average was 80th in yards/game and 80th in points/game.
The concern is will the defense be dominate again, because that is Iowa's path to success. The offense seems to just be a placeholder for the defense, in good years or bad. It would be nice for the offense to be a bigger contributor, but it doesn't seem to matter as much. If the offense is a lot better than typical, we'll be in for quite a year.