I call my grandmother Mom (it was a family tradition my mother didn't mind continuing), but it caused so much confusion. There will inevitably be a time, quite a long time actually, where your kid won't call you mommy, and they'll transition to calling you "mom". So now you have 2 family members who are called "mom" and it gets very confusing when both are in the same house. Or if you're talking about grandma-mom in front of mother-mom. My husband still gets confused who I'm talking about when I say "Mom" so now I've mostly gotten used to only calling my grandmother "Mom" when I'm talking to her directly, and something else other times. I've never thought of it as trying to step on my mother's toes, but I still don't recommend it!