How far are you going to have to go in Tokyo if you're craving Mexican? And is there even a decent Mexican restaurant in Tokyo? (I'm guessing the answer might be yes but not many.)
On the other hand, I understand KFC is very popular in Japan which happens to be one of my favorite US fast foods if I really need to choose.
Some types of fast food appeal to me more than others. I'm actually not much of a cold bento box fan in general; I usually end up picking at them at conferences. But I like a lot of Japanese food generally.
You'd be surprised.
https://www.tripadvisor.com/Restaurants-g298184-c29-Tokyo_To...
Tokyo has insane amount of good restaurants.
I don't think it's just a matter of density. Folks in Tokyo, like folks in New York but perhaps even more so, are willing to try a variety of foods and care about doing them authentically.
I really like some Xinjiang and Hunan dishes, but they're next to impossible to find in the US despite "Chinese" food ostensibly being common.
To have decent restaurants from a certain ethnicity, you do need some significant population of that ethnicity, and at least some of those people who are lower or lower-middle-class. Having moved to Seattle from New York, the Chinese food here is much worse; Seattle has a large population of Chinese people, but the vast majority of those are college-educated and working at tech companies, and are not a likely demographic for opening or running restaurants.