Americans being dumb and fat is literally an internet meme now. I mean look at Donald Trump? Look at how popular he is in the last voting. When the top politicians in the whole country is so infamous for being a sexist, buffoon, egomaniac and running for president, probably solely doing it for the money, you will be surprised on how the world view America. People seem to love him for some reason because he is the closest you can get to the reality TV in the boring politics. Continuing on the topic of politic, they voted George W. Bush, twice while at it. Americans seem to like their share of politicians that ruins their country.
People seem to hate on Obama too for some reason, but that is within America as far as an I can tell, considering what he did all this time is getting blamed for what the previous administration has done to the country and doing a really well job in fixing it too. So yes, if the world's see your leaders that represents the whole country as stupid and seeing how the achievements of the current president are downplayed, it's obvious why Americans are seen as stupid as well. Did I mention their gun control? The pro-gun logics for keeping their guns are wonderful.