It's not that people are getting dumber, it's just that the more people enter school and go for "higher" education levels the lower the teaching standards have to be. I mean what was the percentage of the population that finished high school in the 50s, in England? And what is the percentage now?
In fact, I'm betting people are getting smarter not dumber, even if just marginally. A not very smart guy in Portugal, in the 30s, for example, would go through, at most, the first 4 years of school. Now, the same guy would at least finish 9th grade, probably the 12th as well. That has to help something, even if the difference is almost negligible.