I’m pretty sure it does help, both. How much depends on your physique but air resistance is definitely a factor. Well, definitely my opinion at least.
When I was skydiving back in the day aero was everything, we’d wear different material suits to each other on formation dives - heavy weights would wear a more cotton type suit whereas the lighter guys would wear slick suits, the idea being to fall at the same speed as each other. Flatten out your body from arching you fall slower than the others, reduce your mass by tucking arms in or even standing up straight you fall faster. Translate that into karting if you present less body mass to the airflow you’re creating with forward speed, less resistance against that airflow means higher forward speed. It’s not much, but it’s there.