Define new addition to literature. Whilst I agree the term "Mary Sue" is a relative new term, springing from the Star Trek fandom if I'm not mistaken. It is really not all that new of a concept. For example, take a look at some of the Gothic novels, and I would begin to debate that there are some clear Mary Sue's in them. The fact is that history tends to filter away most of the poorer quality works, and much of what we see now are the stories that have stood against time's ebb. Furthermore, I would submit that Mary Sue's are actually just more easily identifiable within fan-fictions due to the nature of the craft. When working with already pre-existing and beloved Worlds it is rather easy to find oneself falling into common writing traps.
Back to the topic, I would like to believe that are many well-defined follies within each of the mane Ponies that they're not Mary-Sue material, yet if they keep pushing the Characters traits they could soon inadvertently fall into this pit of mediocrity.