I share your views, DOM and carlota.
I personally wouldn't have many problems if a princess (MM or other) took a few days/weeks to travel (study, spiritual, sabatic, whatever) as long as I had the feeling she was working hard when at home. By working hard, I mean attending events and all the things other princesses seem to do. But that's not the feeling I have from Mette-Marit.
In general, I think this is not a big problem in Norway. At least that's what seemed to me when I lived there for half a year. Of course I didn't go and asked people about it, so who knows what their true opinion is... but they don't seem to do a big fuss about it. The only person who once talked about MM was a friend of mine and she said she liked her very much but I don't know how much she follows MM or wheter she knows about her trips or not, so...