I agree with LeeLee-- it's her term of endearment for you. If she's always called you this, pronounced this way, maybe you can consider that whenever she says it she is, without realizing it, calling you what she called you from babyhood. On some level it gives her comfort and normalcy -- that's who you are to her. Of course, you know your own kid best, so if you truly think she's doing it now on purpose to annoy you -- you'll have to handle that. But if you just dislike it, and she's always called you this and isn't doing it intentionally to annoy -- like LeeLee says, you could try to love it, if you can see that it makes her feel secure.
That doesn't answer your question, I know! Sorry! Maybe this is also a bit regional or cultural -- if your own mother was "mom" or "M." you might be preferring that your daughter use that because it was normal for you to say yourself. In our family, mothers were always "Mama" -- mine was my Mama, her mother was Mama to her, and when my grandmother talked about her parents, she had a Mama too. So I am Mama (not "muh-muh," though). I would not want to be a mom or M., myself, but it's regional and cultural and what a family prefers.
If you can't get past it, maybe you and she can come up with nicknames for each other based on something -- maybe she has a favorite kids' book where the mother character is called something you would prefer (all I can think of right now is "Marmee" from Little Women, which would drive me nuts, but there must be others....) She might see that more as a fun thing to do, and might take it better than you saying you dislike what she's always called you. Just a thought.