I went to three different "Mexican" restaurants while I was in Japan. One was an El Torito and it was an El Torito. One was a burrito place that was basically a Chipotle knock-off that was foreigner-centric since the menu boards were primarily in English and the guy behind the counter was also a "gaijin" (but maybe it was just marketing to make it more "exotic" to Japanese customers - the line often blurs) and it was a Chipotle. But one place, the one that looked the most "authentic" from the outside, was just absolute garbage. I can't remember the specifics but mayo burritos are a distinct possibility.
I often made tacos at home using taco seasoning and tortillas that were both available from one of the normal supermarkets nearby (so not a specialty import place). Those tortillas were incredibly expensive - I think something like 500 yen for a 5-pack, or like 85 US cents apiece - but worth it for an occasional treat and you could make it with the 50% beef/50% pork mix that was much cheaper than the 100% ground beef without the taste being ruined too much.
Taco rice is a thing in Japan. It's basically taco-seasoned ground meat on white rice, sometimes with other taco fillings like shredded lettuce, cheese, tomatoes, etc, but no tortilla. I assume that's what most of that taco seasoning packets the grocery store sold was being used for. I've never tried it, though.