A name that's pleasing to the ear. In answer to your question, I would think it slips under the wire of being assigned soley to a particular decade. I've got my name books out today because I'm trying to decide on some shortlist names myself so it's hardly any trouble to add some quotes from them. The reasons I think it's fairly non-faddy are:
i) It is an ancient name. Melania was used by both the Greeks and the Romans.
ii) It is a saint's name. According to the Oxford Dictionary of Names, it was the name of the two Roman saints of the 5th century, a grandmother and a grandaughter. St Melania the Younger was the daughter of a patrician family. It states 'she led an austere and devout Christian life and, on inheriting her father's wealth, she emancipated her slaves, sold her property, and she gave the proceeds to the poor. She also established several contemplative houses, including one on the Mount of Olives, to which she eventually retired.'
iii) It has been established in this country for a long time making it a lot less faddy than many names. It has had an ebb and flow of popularity but it didn't zoom in from nowhere in the late 20th century. The Oxford Dictionary of Names says that 'the name Melanie was introduced to England from France in the Middle Ages.' The Collins book of First Names states that it was introduced in the mid-17th century.
I'm inclined to go lean towards the first reference however, the point is that it's been used in England certainly from the time of the Magna Carta to the reign of Charles I
She doesn't have to be called Mel if you don't want her to be; it's the same argument with Elizabeth, Abigail, Catherine and other names and their diminutives. I know a Melanie who's often called Anie if that is any help.