You mean like how no one in America knows who George Washington was? It's thoroughly established that Marth, the Hero King, is an important historical figure in FE13's setting.
My only gripe with the "pretending to be Marth" thing is that most people in the game don't know who Marth is, or know that Marth should be long dead. . . unless he/she thinks that their Marth, and they are not. . . still a girl pretending to be Marth would be more interesting. We can always use another awesome bifauxnen.
As Shun said, the problem is entirely different. If some guy were to show up today claiming to be George Washington, it's not a matter of not knowing who Washington was or that he died two hundred years ago, but that he'd better have some damn good explanation and reasons in order to expect anyone to believe him.
Edited by Othin, 29 March 2012 - 01:32 PM.