They're both wacko and both could do virtually anything they wanted. Emperor joker took mr. mxyzptlk's powers and totally fucked up DCU. Drove gods (phantom stranger, ganthet, zeus, shazaam, highfather) insane (all of them were saying 'wazaaaaap!!!!'), killed batman and brought him back to life again and again just to kill him another way, shaped the entire universe in his own image, etc etc etc... Bruce almighty is a guy who was given god's powers for a time... Oh and interesting thing is that i didn't know htat mr. mxyzptlk was capable of that much power... aside from the 'Quintessence' (cosmic guardians of the universe) Joker seemed to even taint the Spectre himself cackling madly. How powerful IS a 5th dimension imp?