If you feel comfortable calling her mom and she wants to be called mom then I say go for it. I have been married for 28 years and call my mil by her first name.
Not until you are married and even then, having had this long relationship w/ her I'd probably just keep calling her what I always called her.
I am getting married in March & I will continue to call my MIL, by her name......sometimes I tease them and call my future In-laws "Mom & Dad"....but mostly call them, by their names (it's what I am used to).....when I was married years ago, I was the same with my 1st In-laws (I still talk to ex-in-laws to this day, I adore them! & they are my dd's grandparents)
Kiki (hit my magic age of 45 and no longer TTC),but mom to a beautiful teen DD & 2 angels in heaven & married to my best friend
I would not feel comfortable calling anyone but my mom "Mom." I've called MIL by her first name from day one, and DH and I have been married 20 years. However, there aren't any rules about this. You have to both be comfortable with the name you call her. If one of you isn't cool calling her mom, then the default is to use her first name.