Feminism is awesome. People being allowed to wear whatever they want despite societal restrictions is awesome. Relegating female characters to repetitive roles in historical films that continually focus on gender as their primary identifying character trait? Not awesome.
If you are talking about "Brave", Merida's primary character trait is that she's, y'know, brave.
The people making this all about Merida being a girl are the public. Merida being a girl in the film is part of her character. Pixar's had people going "why don't you make a film with a girl lead?" And then they make a film with a girl lead, and people make a big fuss about how she's a girl. Like how people made a big fuss about Tiana being black, yet The Princess and the Frog doesn't slap people in the face with race issues. They are mentioned, but are fairly subtle.
And lets not get into the unoriginality debate, really, let's not. I don't see how it degrades the main character to make a movie that has a plot which involves her being a girl and the historical role. Being a girl is far from Merida's only defining character trait. She's also brave, good with a bow, and really, really ginger.