Of course it's never okay to be disrespectful. While dating, my DH introduced me as Trish to his daughter, and for a while that's what she called me. I would've been ok with whatever made her comfortable... within reason. Trish, Momma Trish, Ms. Trish, whatever... But she chose Momma. And I'm ok with that.
Also, you say children only repeat what they hear. Well in my case, DH and I don't have any children yet, so there was no one around to call me mommy for my SD to pick up on. She just realized that I'm her StepMOM, the mom of my household, the woman who's married to her father. I don't think that's a difficult concept for her to put together.