The biggest problem is that the NTGs are heavy per kw and don,t produce much power to efficiently halve the transfer time you need alot of dV and to do that you need mass efficient ion drives, but mass efficient ion drives need MWs if power to carry weights in the 10s of tons, particularly if you are talking about a mars lander abd return vehical. At least to leave Earth orbit efficiently from LEO you need a traditional rocket to take advantage of LEO velocity, NTGs are not it.. You need something like a whole Falcon-9 except with vacuum engines. In terms of power production per kg, the new lightweight panels are better.