Disagree. Japan got hit with over three dozen nuclear weapons, which even accepting that the payloads aren't in the megaton range, I'm guessing at least a million people were killed total (OTL Hiroshima and Nagasaki caused about 200,000 deaths), much of the leadership has been decapitated, radiation is going to ruin crops for years to come, and on top of that Japan was surely hit with large amounts of conventional weapons and "normal" biological agents. This isn't even getting into the poor Stone Dogs roll.
Well, I agree that Japan is a likely candidate for being "worst off" as of March 1982 ITTL, i.e. before Stone Dogs really hits. I am just not sure.
However, I doubt that the leadership has been decapitated. The nuclear bombs are "only" 3-4 stronger than the orbital kinetics that was expected, and this is a grim world that looks at OTL 1960-bunkers thinking "amateurs!" Either Japanese leadership is making a deliberate example of being willing to die along with everyone else (which may not be totally unrealistic given that this is a resentful militaristic Japan we are dealing with) or they are tucked safely away in underground bunkers designed to survive WMD attacks.
Also, long term radiation effects from those two bombs OTL seemed to be small in comparison with the direct deaths. The Japanese government OTL recognized some 650.000 被爆者 (literally"explosion-affected people") from the bombing of Hiroshima and Nagasaki, but only around 1% of them have ever developed any illness caused by radiation. In fact, it might be argued that the biggest negative effect on survivors in the vicinity of the bombings was the fear that other Japanese had of them being affected by radiation (in terms of causing birth defects and radiation being contagious). Even with 40 Hiroshima bombs, we are still only talking a fallout effect 10 times less than Chernobyl and roughly the same as Fukushima - and the latter did definitely not cause mass starvation in Japan or "ruin crops for years to come".
So, yes, Japan has probably suffered losses of 1-2% of its population. Still, other nations might be higher. Depending on the success and heavyhandedness of the invasion of Centroamerica and subsequent occupation, you could easily get above that.
Edit: I forgot to add, according to the climate models, that I know of then the nuclear bombs will in conjunction with the orbital kinetics and other bombs very likely push enough soot (5-10 mt) into the atmosphere to trigger a temperature decrease of 1-2 degrees C (more in the midwest and central Euroasia, less on the coasts), or about a 20% reduction in crop production. So yes, we will likely have a starvation scenario, but Japan is among the least likely to hit by that.