The cape is interesting actually and one could argue that is is a stylized produce from a previous, pre-modern, fashion. Of course, Superman's cape is nothing like the cape that would be used in 17th Centruy Delft, or even London. It has evolved in Comics as a fashion and design element of its own, and it is strictly a matter of asthetics.
It is a little bit like the customes that the Dutch used in the 17th Century in their portraits, with large flurries of ruffles that marked fashion from the 16th centruy. But the Dutch loved it in their paintings and even modernized it for customes.