No, I think millenia of self imposed isolationism has forged the Japanese character. If your hypothesis is indeed true then you would share the same characteristics as cultures from other parts of the world that are prone to frequent natural disasters and you would therefore have no problems assimilating with them. We all know this is not true so I guess that pokes a rather large hole in that theory.
First of all, Japanese isolationism was under the Tokugawa shogunate, so it lasted more or less 200 years, certainly not millenia.
Also I can't see why sharing the same characteritics as another culture means having no problems assimilating, there can be problems in assimilation due to historical reasons for example, or specific cultural points which are not shared.
Finally, just because different populations react differently to a certain situation doesn't mean that situation has no effect on them. The effect of climate on a culture's character is absolutely real, but it doesn't necessary work in the same way in different contexts. The japanese character has been forged by many different factors and it does seem reasonable to assume that something as important as frequent natural disasters is an important factor.
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