I went looking for the pronunciation of the deities name.
I found this following page with the pronunciation, I however do not know exactly how correct it is. However, hopefully this may help you with pronunciation, or yes with coming up with a nickname.
And to help answer your next question, I see nothing wrong at all with using a nickname for any deity. As long as you know which deity you are addressing, I see no reason at all why said deity would not be aware you are indeed addressing them.
Or that is to say- the Deity will know you mean to address it, by manner of yourself having in heart and mind what it represents, regardless of what you call it or refer to it as, I feel.
Hope this helps.
I see no reason why you can't come up with a nickname, if you either have trouble pronouncing the name, or if you would simply prefer call it something else.
Edit: Pardon, and apologies, I forgot to link: https://www.howtopronounce.com/xi-wangmu/
What you will want to press with your mouse on the site is the bluish purple like arrow, with sound like symbols like this- ))) near to the little what appears to be "like" "dislike" symbols one might find on youtube in the small box area there.