I looked at General Social Survey data to see how much IQ has declined in the US since the early 70s when the GSS was started. I set whites tested in the 70s to a mean of 100, so they are the comparison group. Here's what I found:
Mean IQ for the 2010s is 99.6, so compared to 1970s whites, the mean has dropped very little. It didn't drop more since we see a Flynn effect among all racial groups. The negative impact occurs because of the tremendous growth of Hispanics who have a 2010s mean IQ of 94 shown below. (All numbers exclude immigrants since they are likely to do poorly on an English-language IQ test).