Chibiusa always viewed her mother as this perfect, graceful, intelligent and powerful woman. And then she saw herself, someone that was eternally a child, clumsy, and powerless. And she felt that she was somehow a failure, that she didn’t measure up.
When she returns to the past, she gets to see that she is not so different from her mom after all. It’s easy to forget that Neo Queen Serenity has had centuries upon centuries of practice by the time we see her. And to forget that she really is Usagi. This was something that was crucial to Chibiusa being able to grow into her own and be comfortable with herself as she is.