Why is Meimei annoyed by her mom at the end of her second tournament in "The Rules of the Game"?

Waverly is annoyed because her mother associates success with how many pieces she loses, regardless of whether she wins or not.

When the family got a used chess set for Christmas, Waverly had no idea that chess would become so important to her.  She learned how to play chess, and it turned out she had a knack for it.  Waverly was surprised when her mother let her participate in a tournament, but her mother was focusing on how many pieces she lost.  She tried to explain to her mother that it wasn’t about pieces.

At the next tournament, I won again, but it was my mother who wore the triumphant grin. "Lost eight piece this time. Last time was eleven. What I tell you? Better off lose less!" I was annoyed, but I couldn't say anything.

Waverly is frustrated because no matter how successful she is, her mother wants more.  She can win, even in a tournament, and it is not enough for her mother.  Her mother wants her to win by losing fewer pieces.  Waverly also feels as if she knows more about chess than her mother, because she is the one who is playing, but her mother still insists on micromanaging.

Waverly feels the pressure of her success.  The neighborhood celebrates her and she is on magazine covers.  She feels that her mother is too focused on her winning.

But I found it difficult to concentrate at home. My mother had a habit of standing over me while I plotted out my games. I think she thought of herself as my protective ally. Her lips would be sealed tight, and after each move I made, a soft "Hmmmmph" would escape from her nose.

This is what leads to the fight between Waverly and her mother, when she yells at her mother in the street for showing her off.  Waverly loves chess.  She is very good at it.  However, the game is tainted by her mother’s controlling nature, and she doesn’t know how to tell her.