Should I ask my girlfriend to stop singing in the car?
Are you kidding? What reason don't you want her singing in the car?
A stupid reason? Yes, she sings terribly
Just let her sing, it's mean to tell your girlfriend to stop enjoying herself. She'll be upset if you're controlling.
She's annoying and doesn't sing like a professional.
Well she is not a professional so she doesn't have to sing like one. And it sounds like she is enjoying herself so it is mean to ask her to stop.