That's only when you have most of Mxyptlk's powers, and this event isn't happening at that period of time. Besides, you can't even rid the universe of Batman. What would grant you the right to name today Thursday?
A.) I did kill Lobo-Batman. So there. :P
B.) I'm The Friggin' Joker. I make my own right.
C.) I killed you on Day 1, remember*? That makes me the new host.
D.) :P :P :P :P :P :P
*For those who don't know, which is about everyone, I attempted to use my nightly hit to kill thatguy on the first day. These stuffed shirts were too snobby to allow it to happen, but in my universe, it still did. :D