Here are the actual fallback settings. You can see that nb falls back on nn, and nn falls back on nb. But there is no fallback for "no".
We added some information to the Kartographer Help page to explain how you can change your fallback settings. You should get community consensus, but it shouldn't be too hard, since you can change your maps fallbacks without changing the general fallback languages for the wiki. (The lists are mostly identical now, but are separate.) So it should be require less convincing.
As long as you're thinking about changing your fallbacks, you might want to consider including a language like English, which has a lot of OSM multilingual names, in your fallback list, since Norwegian isn't in even the top 25 languages for OSM multilingual data. That should make maps of foreign places look more readable. E.g., here is China with the language set to Norwegian, and here it is in English. If, having made that change, you started seeing English names showing up in maps of Norway, you could use the lang="local" setting, as described here. Good luck, and let us know how it works out!