displays some wisdom and maturity and resourcefulness.

No one makes such a vague request to a witch. Plus, she's lazy and doesn't really pay attention or put enough effort to her lessons. I'll admit she's quite skilled, though.

As rhythmicmagic said, she is very eloquent with words, like how she improvised the speech. She obviously has easy access to education and I don't think she's dumb for her request to the witch, just very very rash.

I thought she was pretty smart with the whole shooting for her own hand thing. And although putting the unknown spell on her mother wasn't the smartest move, I think she did it more out of desperation than stupidity.

She has a lot of knowledge as far as archery, outdoorsmanship, sword fighting and general survival skills, I would also argue that despite not seeming to pay attention during her mother's lessons she probably absorbed enough to actually know about the affairs of her kingdom and its history. Merida is the type of learner who has to really enjoy the subject (or at least learn about it in a tactile manner) before she truly masters it. Despite the business with the witch I think its her age and lack of maturity that make her do dumb things rather than any intellectual shortcomings. She does show a lot of promise though during the final speech and with her mom as her teacher and their restored relationship, I see her accomplishing a lot.

laziness doesnt have anything to do with intelligence

She is very resourceful and does gain some wisdom. However, what hurts her is the fact that she never asks the Witch specifically what the cake will do and just trusts the witch, who is a perfect stranger.

What PL said, and she's pretty crafty like Jasmine.

She is, indeed, very resourceful. She knows how to make a quick speech but very meaningful in spite of the three lords scold her initially. In the first minutes after the prologue we saw that Queen Elinor taught her so many things that is what a princess should do. Her archery skill is unquestionably remarkable at only age of sixteen. And not to mention she is skilled at sword fighting too. She also does perform a bit martial arts when she throws one of the lords to the ground and took his sword. Moreover she is quick-witted, being able to deduce the truth about Mor'du in a brief time and how to 'mend the bond torn by pride'. So, IMO, she's arguably one of the most intelligent DP.