If an alien created us, then that's not God. That's a creature, possibly a "more highly evolved," superior, or what have you creature. Any more than me creating my child makes me her God. "God" in the true philosophical sense means "the ultimate, uncreated ground of all being."
As far as I can tell (and I've looked into it), Mormonism believes that God is in some sense an alien (though they don't phrase it that way). They take the idea that we are created in God's image very literally - so basically, he is a physical man, although a "glorified" man, whatever that means. He achieved a high degree of righteousness on his home world, which entitled him to be the ruler of our world. They believe that righteous earthlings can go on to do the same. They have a saying that goes something like "as man is, God once was, and as God is, man will be."
I once asked a Mormon missionary the obvious question "where does it ALL come from?" The alien/human god theory may explain biological human life, but it doesn't explain the existence of the universe. His answer was that this is an interesting question (how do you explain existence in general) but that it is mysterious and not something he felt qualified to look into. I found that odd, because most other religious traditions regard answering that question as the point of the whole thing.