Merida is known for breaking the princess archetype. She shows that not all princesses are girly and frilly, and like Ariel, she shows that the seaweed is always greener on the other side. Girls dream of being mermaids, and Ariel shows that from a mermaid's perspective, being human can be just as intriguing. Likewise, girls dream of being a princess, and Merida (and also Jasmine) show that being a princess is harder than it looks.
What I also see in Brave is that Merida is not only the anti-princess princess, but she is the anti-tomboy tomboy as well. Merida has "manly" interests like archery and loves the outdoors and is good at climbing cliffs and catching fish. She dislikes girly things like dresses. She isn't elegant and finds manners to be stuffy.
A lot of tomboys in the media are portrayed as strong, independent females like Katniss, Rey, and Mulan. Though they may have a few flaws, they have strong moral character, almost always doing the right thing, the oh-so-noble-and-rightly thing. They're wise and ahead of their time. They see all the wrong in society and turn them upside down. Their acts are heroic and they selflessly save everyone. All in all, they're virtuous.
Merida breaks the tomboy stereotype. She's not virtuous, she's not even brave. In fact, it's her elegant and very feminine mother who becomes surprisingly brave and saves Merida. Not all tomboys are brave and not all girly girls are scaredy-cats. Merida gets scared easily (like the dark) and is whiny, needy and irresponsible. Though her archery skills are on par, I don't recall her fighting. Often tomboy equates to strong. But Merida is hardly a strong tomboy. In fact, she may be one of the few "weak" tomboys we've seen. She's the one who needs saving and depends on her family to save her. She's also not emotionally strong and resorts to selfish manipulation to get her way. Usually in Tomboy-land, there is trouble and the Tomboy solves it. But in Brave, there is peace and then Merida creates the trouble. Merida kind of humanizes the tomboy and is a more realistic version.
Merida also shows that tomboys aren't made in a day. What I love about Merida is her dedication to her sport. When she first learned archery as a child, she wasn't good at it. But after years of practice makes perfect, she becomes the most best archerer in her kingdom. Her excellence and skill are the result of her true love for the sport, not because she's born a natural genius.
The title "Brave" can be quite misleading. As a surprise, the tomboy is not brave. Merida's not what you'd expect in a tomboy, being all badass, kicking butt and saving people. Being a tomboy doesn't mean you're gonna save a country. Merida redefines the Tomboy as well as the Princess. She's simply a girl who has some traits of a regular boy. Just like how a boy is just a regular boy.