My mother is constantly calling herself "momma" to my baby?

I think your mom is being insensitive, but whether it's intentional or not, you're the best judge of that. You know whether there are issues below this. If it were my mom, I'd know it were just excitement, but I've known families where the power struggle reaches back generations and keeps on giving, so to speak. It'll be the same pattern in two generations as it was for the grandparents. Some people do make mistakes like this unintentionally. My dad would always call me by my three older sisters names and then get around to mine. I've been known to call the dog when I mean my child. Ugh. talk about embarrassing. But I don't do it on purpose. Either your mom was so used to talking to her babies like that, that she still just does that, or she is doing it to get under your skin. Could be either. I would simply talk to her about this when your baby is taking a nap. If she is understanding and wants to change, there's your answer. If she brushes aside your feelings and keeps it up, then you know what that means. I definitely wouldn't let her continue to do this. But I don't think you can assume anything until you've talked to her and then you decide how to handle it from there.