First of all the nature of a fairy tale is that it is prose of fictional character. This is what differentiates it from a (folk) legend which is generally at least believed to be based on actual persons or events. The fairy tale usually explicitly omits names, times and places („A long time ago in a place far far away...“). Also it often features fantasy elements (Dragons, Magic etc.). Therefore as a prime source fairy tales are hardly suitable for historic research.

That being said there is a kind of grey area between myth and fairy tale. Myths are viewed as very interesting by some historians - especially origin and etiological myths. However some other historians tend to look down on them as sources seeing them as less factual and out of scope for historical research (A very pointed criticism of this view is presented by Marie There’s Fögen in Das Lied vom Gesetz, where she researched founding myths of Ancient Rome). On a side note, the German language makes the distinction between tales and history but the words are VERY similar: Geschichte is history and Geschichten are tales. Another side note: Sleeping Beauty (not the Disney version) is interpreted as a watered down version of Brunhilds story from the Nibelung Saga - showing the grey area between myth/legend and fairy tale.

The fairy tales we know and recount today are often the result of an oral tradition that was mostly recorded in the 19th or early 20th Century though the oldest French Collection of folk fairy tales dates from 1697 (Charles Perraults, Histoires ou Contes du temps passé avec des moralités). Still methodologically groundbreaking was the work of the Brothers Grimm which spiked interest Europe-wide and led to a lot of national folk tales being gathered and recorded.

The importance of common beliefs and the socio-cultural effects that may be interpreted by the comparison or analysis of fairy tales are in my opinion very legitimate historical questions and topics. However most works I encountered were of either philological, psychological or social (social sciences) nature.

One of the Main reasons for the lack of genuine historical research in fairy tales and myths seems to be the fantasy aspects of myths and legends which tend to give a Kontor historians I encountered a very dismissing outlook on fairy tale research and value as a source.

However what I find peculiar is that these same historians that for example state a fairy tale or local myth should be disregarded as a source if it states „the Devil did this and that...“ or „so the princess used magic to...“ seem very happy to look at ancient sources that contain a lot of virtually the same stuff which they just ignore.

My favourite example is Procopius „Secret History“ - one of the prime sources in Emperor Justinian, his queen Theodosia and Belisarius and his wife. Even though Procopius for example states that Emperor Justinian liked to „sever his head“ and have it „floating around the room“ and explains witchcraft he wants to have witnessed by the persons he describes the „Secret History“ is and was one of the prime sources for description of the historical persons. The quite differing description by the same author in other works is interpreted as propagandistic, the secret book being „more true“. The fantasy aspects are left out as hyperbole. If you applied the same standards to - say - a fairy tale I don’t see how you can come to its unusability as a source.

However in the end the use of fairy tales and other oral traditions will always be somewhat limited to social history and related subjects. It is hard to get dates and reliable facts out of them.