A letter grade in a vacuum doesn't make any sense to me. Stats have their purpose, context has its purpose, comp level has its purpose. For example, I can't ignore that we help a 84 ppg team to 60 points. I cant ignore that JBjr was on the other team, and that we locked him down. I cant ignore that we gave up straight line drives, but IU takes everyone off the dribble - they just usually score more points doing it.
That being said, I have no idea what IU's PPP was, so for the sake of not avoiding the argument, I'd give the defense a B-B+