Let me speak as a parent.
There are two things that would persuade me. If you want a game rated mature, then being mature is the best thing that you can do (accepting that your mom is your mom, and even appreciating that is a good start).
1. Make a mature argument
. There are plenty of parental reviews online that explain the different parts of the game. Take this one for example...
I think you can make a good case simply by reviews on the internet. If your mother is religious there are even fairly positive reviews from religious people. But the key is for you to do this in a respectful, mature way.
If my kids took the time gather up the reviews to convince me that the content isn't really objectionable, and if they explain to me respectfully that it was important, I just might change my mind.
. Parents want to feel respected. If we feel respected then we are more likely to be persuadable particularly if you come perpared with a reasonable case and facts to back it up.. Make sure that you are not hurting yourself by making this an issue of respect (because once my my kids make it an issue of respect, the argument is finished).