Hmm... the way I understand 娘娘 now is that it can be for referring to any female figure with reverence and respect. Emphasis on "can" because the definition of this phrase is primarily reserved for really high-ranking women like empresses and goddesses. One that comes to mind would be a mother goddess.
So when understanding the translation from "Seiga Nyan Nyan" to "Lady Seiga", do note that this "Lady" is really close to the top on the importance hierarchy. She's basically styling herself as a goddess. Wicked.
