This term was originally used to describe the wildly popular "Fifty Shades of Grey" trilogy written by E. L. James, which was tremendously popular within the mummy demographic.
Mummy porn may also be called clit lit or female erotica.
More About Mummy Porn
Considering the rising popularity of female erotica, it's no a wonder that mummy porn has become such a buzzword. The term has been recognized by several online dictionaries, and was even a candidate for the Oxford University Press Word of the Year in 2012.