Mean IQ of college grads dropped 9 points from the 60s to the 90s: The General Social Survey has collected data on a simple measure of IQ since 1974. I calculated means for whites across various degrees and decades. (Black college grads appear to have IQs three or four points lower than whites, but sample sizes are very small, so I didn't include them). Most of the action is observed with a 4-year degree, so I'll focus on it:
Mean IQ--white college grads
The trend is clear with a slight reversal in this decade. I looked at people who graduated during the 1950s and before, but I didn't list the scores since they are similar to the 60s cohort.
America is obsessed with educating everyone, regardless of ability, so admission standards have been lowered. A bachelors degree has not held its value over the years.
Fortunately, the mean score for those with advanced degrees has not fallen, but the mindless compulsion to educate more and more people will likely fix that. In my experience, the only thing keeping IQs up is that colleges cannot convince people below a certain IQ threshold to stay in school.