This is an interesting story only when compared to Alan Moore's last Superman story, where Mxy turns out to be an evil killer like his "cousin" here.
I think it's a good thing that Alan Moore's story is imaginary. I actually much prefer Cary Bates' "Trapped in Imp TV" as the final Mxyzptlk story, where we see Superman and Mxy end their decades of conflict with a friendly handshake, rather than by blasting each other into oblivion.
Looks like the Clark Kent secret identity isn't so secret in the Fifth Dimension. Rather disconcerting, that.
Hmm. My take on the whole Superman-disguised-as-Clark trick was that even after such a blatant give away,
still no one caught on -- not even Mxy nor his cousin -- just like in all those old Silver Age tales. That was hilarious! And, I suspect, intentional on Bates' part.