The character in Mulan might be based on a common story, with real portrayals of what life would have been like if there were a person who did all that she did, but I don't believe she was in fact a real person herself. It is fairly common for folklore and fabled themes to turn into blockbuster movies - just look at the Harry Potter series and even all the movies made from Marvel comics. Where there is a fanbase, there is a movie made for that crowd. It doesn't mean everything that happens in the movie did or could happen - after all, talking dragons are also in the movie, and we know they are imaginary!
None of the Disney movies portray real stories, only "what ifs". That is something to keep in mind when exploring this issue. The story of Mulan is a positive one because it portrays a strong woman who displayed honor for her family and country. Also, since most of the Disney main characters tend to be women on a mission, this story has appealed to all the women and girls who are not conventional and don't act girly. For many, it's refreshing because it is not about a princess, and even boys like the movie because it's not mushy and it has elements of magic. But is the movie "real"? I hardly think so.
However, that's not to say things LIKE that haven't happened in history. I have heard accounts of women who joined the U.S. Army or Navy in order to fight for their country during times when the military was men-only. They posed as men, hiding their gender from their peers. I could see where it would be possible in this country for that to happen - but not in the East where women hardly have a voice, especially during the time era in the movie. Disobedience in any way signified punishment or death. Women were below men, existing only for the sake of getting married to serve their husbands and to have children. Sadly, this is still the case for many places in the East today. Of course, times are changing, so the message of Mulan is inspiring for women who feel oppressed. In that way, Mulan has been successful in drawing attention to the sexist treatment between women and men, and how people need to be seen as people FIRST before addressing their gender (or perhaps race).