The Mendoza line was considered to be the level of minimal competence as a batter required to be a serviceable player in major league baseball. Below that level could not justify using the player irregardless of his defensive ability.
Now, if you wanted to say Iowa's offense has been at the Mendoza line much of Ferentz's tenure, you might have an argument there.
As far as the whole team is concerned, I'm not so sure the argument applies. Iowa's performance level has long been used as goal level for what an average team could achieve, given somewhat limited recruiting base and not being anything close to a blue blood program. This was as true during the 90's with Fry's team as Northwestern, Wisconsin, and Purdue were trying to move things up a few notches. Under Ferentz many of the teams on the move from below Iowa have sited Iowa's program as an example of what a competent team could achieve.
You're conflating competence with minimal competence. There are a handful of teams below Iowa's level, and handful at Iowa's level, and four teams definately above.