I was in Japan this year and didn’t see any Mexican food in the 3 major cities I visited. I’m not saying it does not exist but it’s definitely not common if it does.
That also makes sense, though. Food in Japan, even the foreign food, has a specific palette it’s targeting. Mexican food is extremely different from Japanese food.
Typically, any food that is introduced to a new culture is successful once it’s adapted to that cultures palette. Any Mexican food being successful in Japan would likely be more akin to the Mexican/Asian fusion places we have in the States than traditional Mexican food.