Sadzzz,
As a teacher I've had female students with considerably long hair who were very intelligent. I've seen average students with long hair and short hair.
I have a cousin who is almost (if not already) 6 feet tall with gorgeous long hair. I have another cousin, pretty enough to be a model, with very long hair, is about 5 feet 7, and was a straight-A student. My sister, MashaAllah, has long brown hair with natural red highlights and it's always so shiny. I envy her cuz she never oils it. And she's very intelligent and a med-school student.
I've also heard that girls with long hair won't grow. Not true at all. Height is a polygenic trait, it's controlled by many genes, which results in variations in height in a family.
Your hair cut/style can make you look taller/shorter/slimmer/fatter/older/younger. On a smaller person...long hair can weigh them down and make them appear shorter. This idea could have started the myth.
There's no correlation between hair and intelligence. I wonder why the myth even started. Do you think because people would assume that girls with longer hair will receive more attention regarding their hair...become more vain....obsess over their looks...and thus not focus too much on their studies? And end up being less bright? Maybe such a view perpetuated this silly idea.
Whatever the reason.....there's no link. If you as a parent observe that your daughter is having a hard time managing her hair...then go ahead and cut it. If your daughter loves her hair...let it be. I say this because as a kid, I fantasized about having long hair like Heba's. And my mom always had my hair either in a boy cut or shoulder length, lol.