Supergirl has done a pretty stellar job with how it has handled a majority of the comic book superheroes and arch villains it has brought to live-action, and tonight delivered viewers' first peek at the classic Superman baddie Mr. Mxyzptlk . And not just the once during the episode's end, but also in the post-ep preview for next week's bonkers-looking installment, titled "Mr. & Mrs. Mxyzptlk."