I feel the urge pretend I'm bad at math to talk to my crush.
If you're not bad at math, why do you want to pretend to be?
She is also bad at math so we have this in common she may like me better.
You should be proud that you are good at math! Instead of lying about how good you are at math, maybe you can offer to help her. She'll never like you if she finds out you were lying to her.
But I'm worried if she thinks I'm too smart she will lose interest to me.
Don't belittle yourself with the accomplishments that you've made. Being upfront and honest with her is the best way to go about getting her talk to you. Starting a relationship on lies is never good.