Yeah, nuclear-thermal is low thrust because you don't have the rocket exhaust to cool your system. You have to deal with the waste heat and that severely limits your power to weight ratio. I think all low-thrust KSP engines are way, way above real world numbers (necessary as the game engine can't handle burns in time warp and players aren't going to be able to plot them without MechJeb type assistance anyway) but I'm not sure on the nuclear-thermal stuff.