Mom--unless I really needed to get her attention, then I would call her by her first name (it always worked!)--and I don't think it was disrespectful at all-it's all in the way it's done.
My kids call me Mommy--even my 21 and 18 year old sons--by their choice! I actually sign cards/notes to them as Mom.
But it's up to them--and I guess I am a Mommy!