From the start of the relationship with Mette-Marit Tjessem Høiby, the princely couple has always been under an extra magnifying glass and under extra scrutiny. They had to overcome controversy, scepticism and criticism. In the case of Mette-Marit we see -exactly like what happened and happens to Letizia Ortiz in Spain- that the media are constantly looking for the smallest hiccup to tackle.
My explanation for this phenomenon is the lack of distance towards the background of both ladies. They come very close to the common Norwegian or Spaniard. Then the reaction is: Who do you think you are? Sending your kids to posh schools? Shopping designer clothes and holidaying at our expenses? Girl, you think you are "royal" but you once served me a beer in Café Engebret in Oslo. It is the phenomenon of having your head chopped off when you raise too high from the "ranks" you once belonged to.
That is my personal conviction and explanation for the often overly critical attitude towards Mette-Marit (and Letizia). No offense intended. Any other theory for this somewhat picky behaviour towards Mette-Marit is good as it gets. Just speculation.
I don't think that the treatment of MM is in any way comparable to that of Letizia. Yes, MM had to go through a lot of heat before she married Haakon - but so did Maxima. And she was critizised now and then - but so was Mary. And she had the bad luck to have a father who fed the media with stories.
But IMO the Norwegian media never showed that over-critical attitude of the Spanish media. There were actually cases when I thought that the Norwegians are unusually understanding and un-critical (Can you imagine the Spanish couple going on a holiday for 2 month?)
All commoners who married into royal houses were under an extra magnifying glass and are more easily criticized than born royals. And I agree that this might have to do with the "lack of distance". But I think it also has to do with the sex ("Put the blame on Mame") and "media star" status (Why isn't Daniel critizised?, he is a Swede and a commoner)