Frisia was conquered by the Franks under Charles Martel in the 730s and by the time of Charlemagne had been part of the Carolingian realm for several decades. It remained in some ways a distinct ethnic/governmental unit for some time. During the reign of Louis the Pious, for instance, references are made to the Emperor having to send officials (abbots and counts) to bring "disobedient" Frisians in line. That being said, the Frisians were fairly well integrated into the Carolingian world.

The first Frisian law codes were issued under the Carolingians for instance. Dorestad, the premier emporium (trading port) was also located in Frisia on the Rhine delta, quite near to Utretcht, which was also an important administrative/ecclesiastical center. Another important trading site, Walcheren (Domburg) was also located somewhat to the south of Dorestad along the coast. One of the major Carolingian mints was located at Dorestad as well as royal officials collecting tolls and trade. Important ecclesiastical institutions such as St-Denis had a vested interest in the trade port as well.

In terms of Nijmegen, yes there was an important Carolingian Palace (as distinct from a castle) located there, there is no doubt about this. The Carolingian Annals have numerous references to visits and assemblies held there. In 777, for instance, Charlemagne celebrated Easter there, likewise in 806.

Nijmegen's location made it an ideal central location, close enough to travel easily to other important sites in Neustria (i.e. Western France), Austrasia (Western Germany, the Carolingian heartlands), Frisia etc.

To doubt Carolingian influence on Frisia seems very strange to me, possibly it comes from some sort of nationalistic font?