It's not too hard to believe, no, but someone like that doesn't usually rise to power in a major political/military institution. Regardless of his talent for warfare - which is debatable in Kefka's case - if he behaved like that he wouldn't have been taken seriously, and would've been an embarrassment to the emperor.

Also, there's just a hint of "OMG IT R KEWL HE R LIEK TEH JOKOR" in that portrayal