The writer of the story did an interview and said that mxy has always been an avatar of the writers to do what ever they wanted to do with no consequences. It's his character to be up, down, sideways, and everthing in between if he wishes to be.
Fortunately no matter how many times the writer says "My character will now leap out of his comic and kick your ass." it will still never happen since he is a fictional character.
Well okay but if you truly believe that while reading a comic there is a chance that mxy will leap out of it and start joking with you, then you are probably insane and should seek treatment.
Again Mxys powers have never been any different in any course of reality. Galactus is a different character altogether. In alternate realities he has been shown to be completely different while Mxy never has. Galactus also depends on how much bse energy he has in him. Mxy's power level always stays the same. he isnt dependent on anything.
If you cant prove Mxys powers are different in any of his stories then its all relative. Thats how I see it.
Correction: He didn't smack an avatar of the writer, Mxy and Bat-Mite just went to the writer's apartment or office or whatever, but they said it was a weird place so they went back inside the comic
In JLA, it was explained that the imps see everyone else like the artist and writers do. As two dimensional beings. The writer of world's funnest tells us in an interview that Mxy is the writers excuse to do what ever the hell he wants to do in a story.