I'm Norwegian, and we're down to 7th place, but I've lived half my life in the UK, and I can tell you that Norwegians will complain just as much as people in the UK, if not far more whenever I visit family.

It's the national sport. In fact, if you tell most Norwegians about coming 7th on a World Happiness ranking, odds are high most of us will complain loudly about that too, thinking we deserve a higher spot, and that there's no ways the Finns and especially Swedes can be higher than us on the list (being beaten by the Danes is okay).

In other words, I don't think this matches cultural acceptability of complaining at all. Norwegians will express dissatisfaction loudly and about everything (don't get a Norwegian started on the quality of the roads, or taxes; at the same time, don't get a majority of Norwegians started on "American conditions" or the political parties that might bring them about by lowering taxes), while still generally living pretty great lives.