I don't have any data that bear directly on the question--and, no, I don't want anyone to put Nancy Pelosi forward as evidence--but we can see how Italian Americans stack up to other white Americans, and it is true that most Italian immigrants have come from southern Italy.
Using GSS data, I set the white native-born American mean to 100 and calculated mean IQs (based on a vocabulary test) for each country of origin (sample size = 12,522). I also omitted Jewish Americans since their high scores can affect results.
Mean IQ--whites only
Other Canada 99.2
French Canada 98.7*
Puerto Rico 94.2*
American only 91.9*
*significantly different from the white average
Italian Americans are basically in the middle. There are two southern European countries with below-average numbers--Portugal and Spain (many of these may be Latin Americans who identify with Spain)--but such is not the case with Italians or Greeks.
Once again, this doesn't really tell us much about southern Italians. Long-distance immigration tends to select for more intelligent people, and many Italian immigrants, perhaps a significant percentage of them unsuccessful and less intelligent, returned home. We can say that even if Lynn were right, Americans of Italian descent are not out of the mainstream.
By the way, see how low the average is for white Mexican Americans. From what I've seen in GSS and ADD Health data, Mex-Ams who describe themselves as white are not much smarter than those who say they are not white.