He changed his name more than Kelly Clarkson.
John J Mellencamp is his real name. According to his sister the middle initial is something John added at some point. His parents didn’t give him a middle name or initial.
He became Johnny Cougar because studio execs didn’t think John Mellencamp sounded cool enough. He hated it. Especially when he didn’t know about the name until he saw the album cover art.
He kept the Johnny Cougar name for his first two albums. Somewhere on either his third or fourth album he was billed as John Cougar Mellencamp.
At some point he dropped the Cougar name completely finally being billed as John Mellencamp.