Is it normal my mother in law calls herself 'mom' to my dog?

I have a dog since last year. My mother in law sometimes walks the dog for me and my partner when we are out and not able to walk the dog. She had my dog in her house twice when my partner and I went on holiday and we couldn't take our dog. What bothers me alot is that she calls herself "mom" to my dog. For example, when we picked the dog up after our last holiday she said things like "You've had such a nice time with mommy, haven't you?" "Can mommy have one more kiss before you go?" "Mommy loves you." It really really really pisses me off so bad and I've told my boyfriend that it really bothers me but he says it's my problem. He doesn't want to talk about it with his mom because he says that it's my problem and not his. I just think it's really messed up that she calls herself mom to my dog. Just wondering what other people think ....?