The figurative language used in the statement "You're a bike. The other kids are cars." is a metaphor. A metaphor is when you compare two things without using "like" or "as" to help explain something.
Mia's mom means that Mia is unique and different from the other kids, just like a bike is different from cars. Bikes can be special and have their own charm, but they're not as fast or popular as cars. By saying this, her mom is trying to tell Mia that it's okay to be different and that she has her own strengths.
This statement makes Mia feel sad at first because she thinks being a bike means she’s not as good as the other kids. But it also makes her think about her own special qualities, which can help her feel more confident in who she is.