Why are daughters mean to their mothers?

From personal experience I would say that there's no straight answer to this question or not only one answer, it depends on mother's and daughter's personality, behaviour and age.

Thankfully, this was the only time she was mean to me. She's a teenager now. Of course, at times, she can be a little rude and disrespectful to me but, to be honest, I'd be worried if she weren't, adolescence is a tough phase for children.

At a very young age all children can be mean, harsh and cruel when their demands are not met. When, at the age of 5, my daughter asked for a sibling and told her that I could no longer provide her with one, she shouted at me that if I had taken better care of her elder sister she wouldn't have died. This stubbed me, but even at that point I felt her own pain and the need to blame someone. So, maybe, daughters are mean to their mothers when they feel pain, anger, fear…

Neither China nor Egypt, the largest construction visible from space is located in Europe. - Farmingdale Observer

Psychology sustains that, generally, children have a competitive relationship with the parent of the same sex while they get along better with the opposite-sex parent(Oedipus complex). Maybe this could explain why a mother-daughter’s relationship can seem tense, especially viewed from the daughter's part.

She's not mean anymore because she knows she can come to me whenever she is angry, sad, nervous, pissed off, anxious, scared, disappointed, bored, confident, in pain, hungry, lonely, excited, heartbroken, in love. So, she comes to me all the time. We talk, we argue, we cry, we laugh, we hug.. there's no room for her to be mean.