You said that "it would be unethical to create a simulation in which the simulated beings were sophisticated enough to be self-aware and capable of significant suffering", but it applies also to having children, so you clarified that the creator has to have a "reason to believe there was a sufficient likelihood of a sufficiently negative outcome" for it to be unethical. So if the creator of the universe doesn't have such reason, your original assumption is not valid anymore, making it perfectly ethical to create a simulation of universe as long as you believe that the net outcome will be positive.