If you consider the quality of education and median iq of folks in this country as a whole .. they're right -- we *are* dumbasses.
But next time remind them -- that if you select distinct select areas of the country (new england, san francisco, seattle, etc.) and now compare the intellect and educational level of it's citizens...we BLOW them away!
These are also the bright geographic areas that are full of creative, intelligent minds who invent the technologies that those foreigners kiss our asses to purchase and adopt.
Just because we're a melting pot, that has a massive population of foreign born citizens from 3rd world countries with little to no education -- doesn't mean everyone is now uneducated and can barely read/write proper english.
Saying "americans" (with it's wide-varying degree of people on both ends of the educational scale) are stupid -- is akin to saying "All europeans are stupid". Without taking into accordance Europe has countries, cultures, races and obviously not all are the same.
(sigh...you'd be surprised how much i have to argue this point with arrogant brits who continually decry how americans are over-rated in the intelligence part).
Easy version: next time someone says Americans are stupid. Just say "no. You're stupid. Stupid".