This is a simple plot of total win percentage each year, colored by head coach. I didn't know how to handle ties, so those are counted as half wins.

It looks like Harbaugh has done better than the other recent coaches and as well as the historically great coaches. My guess on why he's viewed as under-performing is that Michigan's primary rival Ohio State has just been legendary during his tenure. Alabama, Clemson, Ohio State, and (to a lesser extent) Oklahoma have dominated football during his tenure, so his 10 wins per season aren't as impressive by comparison.