More recent issues have tried to consolidate Mxy's history with Superman a bit -- he essentially let Superman beat him over and over again because he thought their 'game' was fun.
We also know that Mxy doesn't like to go around f*cking with reality on a vast scale, because he doesn't think that's very fun either.
__________________
"I am tired of Earth. These people.
I am tired of being caught in the tangle of their lives."