Carbon fiber is a pretty weird choice for a reusable rocket. Unlike aluminum, it has basically zero tolerance to heat since the fibers are embedded in a matrix made of a polymer glue compound. That's why they call it fiber reinforced plastic.
SpaceX already uses carbon fiber on their rockets in the interstage and payload shroud. They do provide some insulation: they're covered in cork. But most of the rocket body encounters extra low temperatures more than high temperatures.
It won't be used for the rocket but for the Mars transport spaceship.