Unless there is some sort of article written about it out there, probably the easiest thing to do would be to look at how many events the UFC holds in a country, especially PPVs.
USA, Brazil, Canada, UK, and Australia/New Zealand are probably the biggest, most consistent places they hold events. Russia and China are probably the fastest growing thanks to Khabib and Zhang becoming champs. The UFC has also built a Performance Institute in China. They had one planned for Mexico City, too, but I'm not sure where that one stands.
Also, it would be impossible to tell the story of the UFC or MMA without Japan and PRIDE. They haven't held an event there in many years, though. Having some experience with Japanese culture, I could definitely see Japanese being turned off by all the shit talking that is so prevalent in the UFC. Also, Japanese people like sports, but they are mostly interested in Japanese people playing sports. There aren't so many Japanese fighters in the UFC any more for whatever reason. So they might still be into MMA, but not the UFC in particular.