Q: Is carbon fiber conductive?
A: Yes. Unlike fiberglass, carbon fiber is mildly to moderately electrically conductive. It's not a good conductor like copper or aluminum, but it's far from an insulator.
Q: Does that matter for a mast that's just holding up my antenna?
A: It can. If a wire antenna runs directly against or taped to the mast, the mast can pick up RF energy from the wire rather than staying electrically separate from your antenna system. At low power, less than 100w, this effect is negligible. At higher power — especially with an amplifier and a tuner — enough energy can couple into the mast to heat it up.