Maybe she's mad because of a bad ending to her favorite story.
She might be angry because the characters in the manga didn't act the way she expected. Another reason could be that the art style changed in a way she didn't like.
The manga girl could be mad because someone spoiled a plot twist for her. Or perhaps she didn't get the new manga issue she was waiting for.
Maybe she didn't get what she wanted, or someone upset her.
Maybe she didn't get what she wanted or something bad happened to her.
Maybe it's because of her expressions or the situations she's in. Sometimes, the way the artist draws her can give that impression.
It could be that she faced a setback or had a misunderstanding. Maybe a friend let her down or she lost something important to her.
Maybe she's had a bad day or someone did something to upset her.
Maybe she didn't get what she wanted or someone was mean to her.
Maybe she's mad because something didn't go her way, like she didn't get the toy she wanted.
Rock Lee might be mad because his training isn't going as well as he hoped. Or maybe a rival outperformed him unexpectedly and that ticked him off.
Maybe she had a bad day or someone did something to upset her.
I have no idea. This is a rather odd and perhaps inappropriate question.