A North Japan would be as crazy or crazier than the DPRK for the same fundamental reason; they are a divided part of a previous whole (whose citizens never wanted to be divided btw) that would be in extremely direct competition for their basic legitimacy with the other part of the whole, a larger capitalist state that is a better deal for its citizens in every way and also far stronger militarily. All of the people in it would want to join the capitalist part if possible because nationalism and money. There fundamentally isn't even a good reason states like East Germany, North Korea, and ATL North Japan should exist.
At a certain point the only way to keep the regime together in the face of that is through really brutal oppression. This is a pattern that recurred again and again in states divided into communist and capitalist spheres. Even when comparing other otherwise comparable communist regimes, the DPRK and DDR stood out as being really brutal.
It is possible you could avoid this dynamic with something like Decisive Darkness where the capitalist parts were somehow more dystopic. In that case North Japan could maybe emulate the least brutal "communist half of a divided former state", North Vietnam, which was an exception because it was stronger and richer than the South and was able to unify before the two could really pull apart.
Making a communist state richer than comparable capitalist ones and then making that hold in the long term is really, really hard, though. That is, in a nutshell, why communism isn't a going thing anymore.