Not only are the Brigadoon costumes no worse than Outlander's, I would say that the Outlander costumes are an even greater distortion of mid-18th century Highland Dress than Brigadoon's.
At least in Brigadoon you have the panoply of colourful tartans which was so characteristic of mid-to-late 18th century Highland outfits.
The kilted men are wearing proper kilt hose and plausible shoes.
In Outlander everyone is in dull brown and grey tartans and jackets, and wearing knee-high riding boots, for which there is no evidence whatever.
Yes there are anachronisms in Brigadoon such as Victorian sword belts, plaid brooches, sporrans, Glengarries, and Kilmarnock bonnets. Though from a later period, these things are all actual items of Highland Dress.
On the other hand the huge floppy Rastafarian bag-hats in Outlander weren't part of Highland Dress of any period.
The one thing that Outlander gets a bit more correct are the shirts. Brigadoon has the Highlanders wearing the pirate shirts so loved by mid-20th century Hollywood (and these in wild colours) which never existed at any time or place whatever.
Another thing besides the costumes is the hair: in the mid-18th century, as it was in the mid-20th, men were generally clean-shaven.
The real Culloden and the Outlander Culloden.

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