I used to be heavily involved in the R/C sailplane hobby. Fiberglass, carbon fiber, and kevlar cloth and weaves were used extensively. I have hollow-molded wings that are QUITE strong and very light. They are molded fiberglass with carbon strips top and bottom over the spar. Beautifully made, in Europe.
The thing is you still have resin. Resin is where the weight comes from, also how many layers of cloth, whether there is a mat or foam or some other filler in between the cloth/resin layers.
IMHO, the minor weight loss and strength gained from carbon fiber isn't worth the difference in pricing compared to fiberglass cloth. Especially in a LARGE object like a car body.
Now, if there are strips installed or patches where extra strength is needed, fine, that makes sense.
The only reason to do carbon fiber throughout is to have "bragging rights".