Both, to be honest. She could have stopped Zim, but her own plan involved hollowing out the Earth, so she ended up being the greater of two evils at the time, so to speak.

[As for what happened with the Moon...]

Apparently he made a bet with my sister over the fate of Earth, resulting in trying to banish the loser to the moon depending on how their bet went. This nearly caused me to get banished there too.

Another time, he apparently got into some kind of turf war with some aliens called the Planet Jackers who wanted to throw Earth and our moon into the sun? That one I somehow managed to not get too involved with, though I can't remember why I chose to stay out of it that time.