Space acts as a thermal insulator most of the time because radiation is the only means for a spacecraft to get rid of waste heat. Thermal radiators also only work if they can be oriented that they don't face the Sun.

The exhaust of a nuclear thermal rocket isn't waste heat. The heat from the reactor that heats everything that is not propellant is waste heat. Getting that heat to radiators without cooking people or melting/weakening load bearing structures is the challenge with nuclear reactors in space. On Earth we use literal tons of water and air to the job.