While Robin Hood was a myth, the era he is based in is not. How accurately is that era portrayed in modern media such as the Disney film, the Mel Gibson movie, the BBC show, and other such forms?
All that can be said is that they're all "made up". As they're all dealing with myth, the producers and writers have a pretty much free rein as to how they portray the era. One can't accuse them of inaccuracy because, essentially, they are portraying a character and a time that can be whatever they want it to be. The "era" that they choose to portray is their perception of an era, not a real era. So they can, again, give full rein to their imaginations. Consequently, the concept of historical accuracy in Robin Hood based film is redundant and irrelevant.