The term "Mary Sue" is clearly delineated in fan fiction, where canon foreigners are clearly visible. It becomes a lot harder to define in actual official fiction, since many of the qualities of a Mary Sue in fan fiction are the qualities of a lead character in official fiction (what distinguishes them is dropping a new lead into an existing world). It tends to get tagged to new characters parachuted into an existing universe and under the pen of their creator. Fans expect Batman to be the uber-man; they're reading Batman, after all.