In the movie, she objects to the sparkly dress because it's too confining and isn't anything like what she wants to wear. In the archery scene where she does wear the sparkly dress, she causes a mini uproar because you can see her corset and underthings as she rips the dress in the process of showing everyone up. Speaking of which, it's the same corset that her mother helps her put on in a painful scene before the archery one and she's told she has to conform to a specific set of beauty "ideals" to be a fitting princess and heir to the throne.
So, seeing Merida in the sparkly dress she hated so much and without her bow and arrows is pretty distressing to me.
It seems like you're over-analyzing this from an anti-political-correctness dogma. The fact is no girl is made to feel guilty for liking sparkly dresses just like no man is made to feel guilty for liking camping. Just because someone complains about the sexism of the stereotypical portrayal is not an attack against the existence of the base archetype, and you are quite frankly being way oversensitive to take it as such. It's the same reason that there's no such thing as "reverse racism": as a member of a privileged group, whatever indignation you feel pales in comparison to the that blacks / women / girls have had to deal with when they step out of accepted social norms.
What's wrong with having a choice? Should all movies show men only doing manly things? Is it shameful for men to be in less than stereotyped roles? The presentation shown to many girls is precisely that they are expected to fall into certain roles. Breaking that mold is not a bad thing.
Isn't the princess in the movie an archer? Why can't giving her a sparkly dress mean: "It's okay to wear a sparkly dress and do archery at the same time! Do whatever you want!"
That seems...more in line with the idea of liberating people from binary gender roles.