I think that's a core part of my feminism. In high school, I do remember trying to "dumb it down" because boys felt threatened by smart girls. Yet, even remembering how I felt, it still troubles me when young girls today do it.
I have grown up a lot in the last 30 years and now I find myself troubled by the labels people put on me. At work, if I am as confident as a man, I'm a "bitch". If I cry, I'm hormonal. If I don't cry, I'm "cold as ice". I find myself fighting others' interpretations of my actions more than acting in a certain way.
At my previous job, there were a lot of layoffs. We have a male and a female director of IT. The male director was laid off (I'm assuming because the female made less money so they saved more by sending him off). The female was forced to convene the whole IT group and gave the news to us. She didn't cry, though I knew her well enough to know that she was sad about what had happened.
The number of times after that I heard that she was an "ice queen" because she didn't cry couldn't be counted on both hands. If she had been the one laid off, NO ONE would have even mentioned that the male didn't cry.
Men and women are absolutely equal when it comes to brain power. It's only what we choose to do with it that makes me loopy. Like you, I become frustrated when I see women doing the "I'm just a silly girl" act, even though I realize that, in a lot of cases, it's a defense mechanism to how awful high school is. Some of us never seem to outgrow high school.
FWIW - high school was brutal for me. Even after trying to "dumb it down" so that boys wouldn't be afraid of me, it didn't work. I never did have a boy friend in high school (I'm 6' tall - and was in 9th grade -and was kind of shy then. I scored in the 99th percentile on standard tests and always got the extra credit questions right in my honors classes) because no matter what I did, I think it looked forced. I got straight A's and spent most of my time in the library when others were talking about the homecoming float. I was labeled a "geek" but even the geeks didn't like me, so I was basically a loner.
I think that it helped me to get over it that it didn't work. I'm not sure how I would have turned out if my attempts to seem less smart had actually worked.