If you really wanted to do pointless stuff with numbers, you could say that fission has an abysmally low "efficiency" because not all of the mass of the atom is

If you really wanted to do pointless stuff with numbers, you could say that fission has an abysmally low "efficiency" because not all of the mass of the atom is converted into energy, with the majority of it ending up as stable fission fragments or neutrinos that zip through the reactor walls. You start with a U-235 atom that has a rest mass of 218,871.5 MeV and only get 202.5 MeV of heat out of it? Pathetic!