That is only true out till (a litle past) Mars orbit. For deep space mission solars performance in terms of Watts per weight quickly becomes pretty bad. The 1/r^2 drop in solar flux eats your lunch there.
Fission is fine out there. And DT fusion has no advantage over fission anywhere in space.
But I tell you what: let's let space programs pay for fusion, if that's going to be where it gets used. They all seem to have much more immediate tech development needs closer in.
