Clearly Mysterion’s power is that he’s terribly mysterious.
Uhmm…he wasn’t even born and raised in the real Cajun part of the state. He grew up on the border of LA and MS. That isn’t even close to being a real Acadian. Anyone from down here will tell you that it was an obvious act.
He is Schrodinger’s Superhero.
It can’t be Kyle because in “The Coon” (S13 E2), Mysterion comes into Kyle’s room and asks for his help.
Maybe it’s a “roving identity” like Captain Universe, where it leaps from person to person, beyond their control.
That or it’s Kenny. 
I like the Kenny theory more and more.