Not sure what culture you are from, but in Poland at least I have almost never heard girls (or anyone!) say anything particularly bad of that sort to boys, but I sure have heard said a lot to girls.
There's definitely still a strong 'boyish' culture where boys can say almost anything they like and it's OK, but girls (for whatever reason) won't. It's not about who's better - it's about what we let people get away with.
Also it's not about thinking. Thinking is the ultimate change you want, but we can't see and know what people around us think, but we can still try to enact standards of reasonable, gentle (non-violent if you will) communication.
I remember from my childhood that boys were always treated as silly clowns with no patience, responsibility and conscience. (Not saying there's no biological basis for this, girls do mature faster as far as I know)
School was much more set up for girls and they were more successful on average.
I don't remember boys saying girls are dumb or the reverse. The stereotype was more about girls being obedient and tidy and organized and serious while boys are messy, forgetful, chaotic and mischievous.
Edit: this last point also makes it weird for me when people equate male with order and female with chaos. I guess the stereotype may have reversed since then? Or the adult stereotype is the reverse of the child stereotype?
There's definitely still a strong 'boyish' culture where boys can say almost anything they like and it's OK, but girls (for whatever reason) won't. It's not about who's better - it's about what we let people get away with.
Also it's not about thinking. Thinking is the ultimate change you want, but we can't see and know what people around us think, but we can still try to enact standards of reasonable, gentle (non-violent if you will) communication.