My understanding is that 思われる, in addition to being the passive form of 思う, can also be used in the sense of "to spontaneously think; to appear".
In North America a skirt is thought of as something a woman wears.
In which sense is 思われる being used here? Both seem reasonable:
Spontaneous: In North America, people spontaneously think a skirt is something a woman wears. Passive: A skirt is thought of by North Americans as something a woman wears.
I lean more towards the first one, since a passive sentence would normally mark the agent with に i.e.