The Brits — including, of course, the Scots, have had an ambivalent attitude towards French fashions and mores for centuries. The French are seen as both refined and effete (among other stereotyped views).
In the context of the books and the series, a couple of thoughts:
Mary Queen of Scots was Queen of France (briefly) and brought a taste for French things home with her to Scotland a couple hundred years before our story takes place. There are close cultural ties between Scotland and France. We will see, for instance, the Fraser connections of Abbott Alexander Fraser and Jared Fraser who’ve both made their homes in France, as have a number of emigre Jacobites. (Interesting trivia: the term “caddy” for the boy who carries your golf clubs seems to originate from Mary’s time. It comes from the French word “cadet” meaning young family member.)
Remember, too, that Jamie spent several years in France. In the 18th century the University of Paris was among the very finest in the world, and Jamie’s father sent him there. Jamie later spent a couple years as a mercenary with the French army.
Terry has stated that in the Series Dougal’s backstory includes time in France, too. This helps account for his aristocratic, refined appearance, including why he wears breeks (or, as Terry says, “trews”) rather than a kilt.
Surely one of the most amusing and evocative lines in the Series comes from the scene where Mrs. Fitz is dressing Claire for the first time. When she expresses surprise at Claire’s bra, Claire explains that it’s French. Mrs. Fitz’s reaction skillfully captures a mixture of shock, titillation, and the view that those French women are likely to do or wear just about anything.