You can get top-quality Japanese food in thousands of small Japanese towns, and top-quality Thai food in thousands of small Thai towns. However, rest assured you will have a very hard time finding (say) Mexican in either. The only places in either country where you have a genuinely cosmopolitan food scene are Tokyo and Bangkok respectively.

What is top-quality American food, and where do you find it?

Not an American, but the last time I rocked up to a random small American town and chose a random small restaurant, I had a really good turkey chili burger [1]. Hardly gourmet, but delicious, and it's quite difficult to find this kind of thing outside the US.

[1] The "Chili Monster" at the Dillinger in Boulder City, NV, in case you were wondering. https://www.thedillinger.com/ (Not to be confused with Boulder, CO.)

In addition to the burger previously mentioned, off the top of my head as "American cuisine" there's obviously barbecue, as well as Southern cuisine, Cajun/Creole, and -- while some might argue -- I'd throw in Tex-Mex and New Mexican (e.g., the state of New Mexico) as gastronomically distinct. Where you find it is, well, all over. You have to do a little research. :) But you can find really well-regarded restaurants around America for those cuisines, across a wide variety of price points.