I could be wrong, but in the Guadalupe apparition, did not Mary appear with the features of an Indian, and was she not wearing a tilma, a garment worn by Juan Diego’s people?
Even in 1918, women were still wearing long skirts, too. Mary’s outfit would not have seemed so strange.
And in Japan, for example, with our Lady of Akita in the 1980s, well, though ‘modern’ Japanese wear ‘westernized clothing’ for work, etc, I do believe that many if not most still wear the kimono.
So I wouldn’t expect the Blessed Mother to appear even in, say, Virginia today wearing a ‘prep’ look of knee length skirt, tights, and sweater-set, even though it would certainly be considered a ‘modest’ outfit’ for a young woman.
Mary is a queen. Her soul and body are united in Heaven, and Heaven is her ‘home’. I think that is the reason that her appearance remains what we might call largely unchanging. In Heaven we won’t have ‘fashions’ the way we do here–we will certainly be clothed in garments when our souls and bodies are united at the Last Judgment (Christ reminded us that we will wear the ‘wedding garments’ that are expected of guests. . .which are as a side note provided by the ones giving the feast). . .
Much as Christ, in His glorified body, could be ‘recognized’ or not depending on the ‘sight’ of the followers, Mary is ‘recognized’ as well.