My favorite Mxyzptlk tale is a post-CRISIS story where he turns Superman and reality into an animated cartoon form and assaults him with various cartoon analogs of Fred Flintstone, Mighty Mouse, the Smurfs, and some sea serpent that I don't remember its actual name.
Cecil the Sea-Sick Sea Serpent, perhaps?
Off on a tangent: wonder what it must've been like to see Mighty Mouse or other superhero cartoons on Earth-1 before Superboy came along; guessing the animators must've thought that a character who could fly and had vast strength was "just a silly cartoonish idea that could never actually happen". For that matter, would love to see what Superman thinks of such superhero cartoons like Mighty Mouse, Underdog, Atom Ant, etc...