I don't think any pro MMA fighter would pride themselves on having a crappy McDojo black belt, mention it in interviews and have it on their UFC profiles.
I don't intend to list "fake karate" at all. The offshoot and sports styles aren't all "fake" and McDojos.
Guy Mezger is 6th Dan and 2x world champ in some vague "Full Contact Karate", which I can't find any specific info on. Was he legit? Fuck yeah!
Chuck Liddell has "Koei Kan Karate" tattooed on his scalp - that's his foundation style. It was founded by some relatively unknown Eizo Onishi guy who has no relation to any modern "mainstream" style. He was trained by students of Itosu and Higaonna though and that's pretty legit if you ask me. And if the style gave Chuck Liddell a solid base to build on then I guess it's not a bad Karate style at all.
A personal example, I myself trained in Fudokan, a tiny Shotokan offshoot:
http://www.fudokaninfo.com/
Fudokan calls itself "a traditional Karate style", differentiates itself from all other styles and is not a member of JKA, ITKF, WKF etc. Sounds like a "fake style"? However, note that the founder, Ilija Jorga, is a legit Shotokan master and 3-time European champion (1986 in kumite, 1976/78 in kata) who received his Dan grades (up to 8th!) from Tetsuji Murakami, Taiji Kase and Hidetaka Nishiyama.
https://en.wikipedia.org/wiki/Tetsuji_Murakami
https://en.wikipedia.org/wiki/Taiji_Kase
https://en.wikipedia.org/wiki/Hidetaka_Nishiyama
I consider that lineage legit as fuck. Fudokan is pretty much oldschool JKA Shotokan since all of Yorga's teachers were oldschool JKA masters. We've had a JKA 1st Dan from Japan visit our dojo a few years ago and he said the training that we do is identical to what they do in Japan.
So, is Fudokan a "mainstream style"? No. It is a "fake style" and "wannabe karate"? I don't think so.
I assume that most fighters who got their black belts in small offshoot styles such as Fudokan wouldn't usually be inclined to specify what exactly that style is called and would just refer to it as "Karate".