There's a never a non-zero negative chance of Superman not running into the loveable and annoying imp from the Fifth Dimension (is that the right amount of negatives???). But boy, when he does, it sure makes a show!
Now the question is, is Mxy messing with his underwear (or lack thereof) choice, or is he just revealing what was already there? I gave you the option to think about what we think the Man of Steel wears under his tights because I couldn't decide myself!