This is a question that I have heard a fair few times. Many eikaiwa teachers come to the conclusion that Japan has a world record breaking number of miserable marriages. Whilst that is conceivable (some nation has to be top!), there are plenty of cultural factors to take into account before coming to that conclusion. One is a tradition of disparaging members of your family in order to appear humble, including the not totally disappeared habit of saying “my smelly wife” as a humble form of okusan. Another is a tradition of taking the mickey out of the man in the house and of the salaryman more generally, as explained in detail by Ian Buruma. As gruff macho dads tend to get this ribbing most, I’m guessing it is a way of evening up the power relationships in the home. The third factor is a comparative lack of safe topics of conversation in Japanese making each one appear like a national obsession, most notably with the topic of food but also with this one. The final influence that I can think of is that many Japanese people seem to think of slagging people off as as Western or British sense of humour, or alternatively find it to be a kind of humour they can easily express in English, and so tend to say those kind of things much more in English than in Japanese.