Hard for me to judge whether those C’s in math will be mitigated by the high school she attends. The school GC will have better info on that than most of us, unless our kids attended this particular high school. In her shoes I would not write an essay about why I want to major in economics and when asked about majors I’d reply that I was undecided.
As a point of comparison, my younger son was pretty sure he’d be an IR major, but there was nothing in his high school activities that backed it up, so he chose to tell a different story. He was a consistent B student in math - but was always in the highest track taking BC Calc as a senior. (Well not quite the highest, there are usually a small handful taking Linear Algebra as seniors.) He scored a 690 on the math section of the SAT. He asked his pre-calc teacher to write a recommendation because he loved him so much. The teacher wrote what I thought was an amazing letter saying that while my son didn’t get the best grades (because he was lousy at memorizing formulas and shortcuts) his teacher thought he had the best math brain in the class, because he was always figuring out how to do problems from first principles on tests. He frequently ran out of time, but found interesting ways to attack problems. I’m sure that letter, helped mitigate all those B’s. He got into Chicago (EA), Vassar and Tufts. He was in the top 6% of a very large, very diverse high school, so not comparable at all as far as rank is concerned.
Good luck to her.