Obviously Mxyzptlk (Peter Gadiot) presents some serious danger for, well, everyone since his magical dimension-bending powers allow him to create endless possibilities for mayhem. From giant crystal monsters and resurrected parasite aliens, Mxyzptlk’s proclivity for violence and destruction is a pretty serious flaw. Plus, he pulled out the whole “nasty woman” thing, which is an instant turnoff. Ick.
Luckily, Kara recognized the many pitfalls of a potential relationship with the fifth dimensional being and immediately put Mxyzptlk on the “do not date” list.