Whilst Norway is not a member of the European Union, they are part of a Bilateral Agreement with the EU regarding air transportation. As a result of this, EU261 DOES apply to Norwegian airlines.
This can be confirmed on the main EU Air Passenger Rights website, which states :
EU means the 28 EU countries , including Guadeloupe, French Guiana,
Martinique, Réunion Island, Mayotte, Saint-Martin (French Antilles),
the Azores, Madeira and the Canary Islands as well as Iceland, Norway
and Switzerland. It does not include the Faeroe Islands, the Isle of
Man and the Channel Islands.
So yes, flights within Norway by Widerøe ARE covered by EU261.