The story might be about your mom's way of comparing you to others or to your past self. By 'pegging' you, she could be highlighting your strengths or areas for improvement. Maybe she does this in a humorous or loving way that makes the experience memorable. For instance, if you are part of a sports team, she might peg you as the most determined player on the team, which is her way of showing pride in you.
Perhaps the'my mom pegs me story' is about how your mom uses this concept of 'pegging' to teach you values. If she pegs you in a situation, it could be her way of making you understand the consequences of your actions. Let's say you made a mistake, and she 'pegs' you as someone who should learn from it. This helps you to grow and become more responsible. It could also be a family - specific term that has a lot of back - story and shared understanding within your family.
It could mean that your mom has a unique way of categorizing or marking your progress in something. For example, if you are learning a new skill, she might 'peg' you at a certain level to show where you are currently at.