Don't know if I'd extend it to _all_ American produce, but some of it does suck very bad indeed, especially tomatoes. I grew up in an area famous for its tomatoes, so my standards for what constitutes a good tomato are impossibly high. The only tomato that's not a complete abomination in northwestern US are those Kumato tomatoes that Trader Joe's has sometimes. On the other hand the United States quite possibly has by far the best apples in the world, if you're willing to pay like $2.50-3/lb - that's how much newer, patented varieties such as Sweetango cost. Bell peppers are about the same. US onions are great (and huge). US garlic is total inedible shit. US corn on the cob is great. Asparagus is great as well, when in season. Some varieties of strawberries are great, mysteriously, even if they aren't fully ripe.
TL;DR: there are things that are better, and there are things that are worse. As with any region, you need to know what the region excels at, and use that for best results.
Generally, I feel that US grocery stores make an overly heavy emphasis on shelf life. You can't have a tasty tomato that also has a long shelf life, but you can have a tasty bell pepper that does.
What you're seeing in Greece and Turkey aren't just ingredients. It's the culture: the understanding of what those dishes are _supposed_ to taste like. That just doesn't translate outside its natural habitat. Take the same ingredients, bring them to the US, have a US cook make you a salad, and you more likely than not you will get shit Greek salad. Same with trying to make Texas BBQ in Greece - you will get shit BBQ.
Outside produce, IMO the US has by far the best beer scene in the world, particularly on the coasts. Just 10 years ago there really wasn't much, and nowadays I see US beers in pubs in London. And by "beers" I don't mind piss water like Budweiser, but real, actual world class beers. It's well past the point where Europe is kinda disappointing, beer-wise.