One reason not to bring it up is that your dad may feel backed into a corner and like he has to follow through on what he said if you tell him you heard it. You don’t want to force that issue.
I once heard my dad say something VERY uncomplimentary about me when I was about your age. I never confronted him and he doesn’t know I heard it. It was over 30 years ago, and frankly it contributes to the fact that I live hundreds of miles from my parents. I am a dutiful child, but I know what he really thinks… But bringing it to their attention that you overheard just isn’t going to go well.