John Cena shocked the wrestling world when he turned heel on Cody Rhodes ... but The Miz didn't bat an eye -- 'cause he tells TMZ Sports he's ALWAYS known he was a bad guy!!

We caught up with the WWE Superstar out at LAX ... and he gloated quite a bit over the fact he called this development from a mile away -- like, more than 10 YEARS ago!!

"Do you not remember WrestleMania 27 going into the main event," Miz said. "Do you not remember all the promos that I have cut on John Cena? All the feuds I've had with John Cena?"

"I've known this forever, but no one believes me because I'm the bad guy."

Despite Cena calling out everyone from his loyal fans to even Ric Flair ... the 44-year-old thinks the leader of the C-Nation won't end his WWE career as a bad guy, and he'll return to his good-guy ways.

Wherever the story goes, the former WWE Champion says Cena's shocking turn is not only good for the business -- but also for his WrestleMania opponent.

"The champion, Cody Rhodes, needs that type of bad guy," he said. "You need that type of heel to go into WrestleMania. Right now there's nothing hotter than John Cena, and no one is talking about anything else it feels like than John Cena."