In John Everett Millais' painting, "The Black Brunswicker," the woman's dress is heavily creased like it was folded very neatly into a rectangle for storage before wearing.
Was it ever fashionable to show creases in clothing during the Napoleonic War (the era of the subject of the painting) or 1860 (when the painting was made)?
According to textile historian Leimomi Oakes there are several reasons why you see a lot of creased clothing in paintings:
Natural fibers are very easily creased and cannot be easily pressed. Princess Diana's silk wedding dress looks extremely wrikled, if Princess of Wales can't keep her dress from getting wrinkled even with modern iron and steamer, they certainly couldn't do it a century ago.
Any garments that require ironing nowadays can be done easily and quickly, but cumbersome garments require a lot more efforts to iron, especially before electric iron.
Dresses used to come neatly folded in a box, so having rectangular creases is a sign of a new dress being worn. Very few sources actually discussed this, so you'll have to take her words for it. There are a lot of examples from the top of my head with the rectangular folds though, for example Baker's Chocolate logo was based on this painting and here is Napoleon's niece Charlotte.
