I think Mayhem just has ADHD (and maybe depression.) People with ADHD and possibly depression act like- and do the peculiar things like he's been doing when they get older. I think he even said a long time ago that he has ADHD and it's been a big challenge all his life.
And, of course, the big thing is he's taking his "Mayhem" persona to a new level. Mayhem isn't just an excited Jason Miller anymore, he has his own clothing style, hair, accessories and everything. Mayhem legitimately looks different from Jason now. He even has his own queer mannerisms.
He has a lot of photos of him dressed regularly, when he's not in character and acting normal, and then he dresses up all krunk and does some Mayhem thing that he posts, then it's back to normal. Of course, a lot of people are taking this as a sign of mental degradation, not just having fun and acting. There was even a part on the Rogan show when he was on it a few years ago when he started to act a little hyper and Joe Rogan, being familiar with him on a personal level, told him to quit acting like Mayhem and act like he normally does, which he did.
I highly doubt he's bipolar or schizophrenic, like people assume. He'd have legitimately been thrown into a mental institution during his encounters with law enforcement if that were the case, or at least spent a few months in jail. Not just had three separate encounters over a four-year period like he's had. If he does have a serious mental illness like that, he's handling it way better than just about any other unmedicated bipolar or schizophrenic person in the world that acts as peculiar as he does. Seriously, if he really is how he acted on that interview, he wouldn't even be aware of reality enough to sign a contract-- literally, he wouldn't be able to sign it.
I liked how he didn't back down on his opponent being Josh Barnett, then he goes like, "Look, I'm fighting Josh Barnett, former heavyweight champion, he changed his name to Luke and lost a lotta weight, but that's not gonna work on me."