They don't even seem that strange as an American. It's just (a parody of) typical conservative Protestant small-town mentality. Much of Scandinavia at the time was made up of religious, Protestant small towns, with most of the population engaged in farming or fishing (and the novel in question was set in such a town). People in small-town and rural America don't like big-city, degree-and-money-having people who "think they're better", either. Sinclair Lewis's 1920 novel Main Street, about small-town American attitudes, has some loose parallels.

I think the same is true even today in the US. Some of the people who're lionized in Palo Alto or NYC would be shunned in much of the "heartland", if they flaunted their wealth or education and came across as thinking they were a "super-achiever". You're allowed to be rich, but you're expected to act somewhat humble and "down-to-earth" about it and not see yourself as better than other kinds of people (especially farmers, who have their own mythos). Warren Buffet, a lifelong midwesterner, is an example of being rich but still sort of pulling that off.