A NTR requires the reactor core to be hotter than the exhaust gas stream/propellant in order to transfer heat to it. Anything over 4000 deg K, structures in the core are going to melt and that would be bad, and that limits how hot and how fast the exhaust will be.
Nuclear thermal is more efficient than chemical rockets but not that much more efficient. It can use readily available mass like cometary or asteroidal ice or gases like methane mined from Titan but if you have access to such sources then simple cryogenic fuel/oxidiser combos like LOX/LH2 produced from ice by solar-powered electrolytic plants are going to be easier to manage and less massive than a reactor-based rocket motor. In such a case the vacuum of space works to your advantage to keep the LOX and LH2 from boiling off too fast.