Asking "who created god" is sort of a weird question. We would need to prove whether God exists or not before we can start thinking about who created him. As far as creating a multiverse goes, a multiverse or universe can be seen as a very large ecosystem. We as humans create ecosystems all the time, whether it be a scientific experiment or even something like a fish tank.
I think that the reason why positing god as the cause is problematic is simply that it is counter to all of our experience to suggest that the complex comes before the simple.
Our experience of the world sugeests very strongly that life began in very simple forms and that they became more complex over time, ultimately leading to amazing organisms like us.
So in our experience of the world the seed becomes the tree, the egg becomes the bird and in an evolutionary sense the bacteria over billions of years evolves into a mouse.
So the original cause/creator if there need be one would logically be more simple than the compexities that emerge later.
If something so simple as the early universe, which was just energy can not come into existence by itself - how are we to accept that an infinitely more complex and improbable intelligent being can?