Superheroes have been mainstream since before you were born, specifically Superman and Batman.
If there was a stigma, The Dark Knight got rid of it. Still, it's not like SW doesn't have its own stigma along with Star Trek. Disney is trying to change that by introducing more female characters into that brand...
Yes, because apparently no one ever made Batman and Superman references in public before IW, right? There are literally phrases in the English language that people use because of Superman.
Superman and Batman precede the Reeve and Keaton movies by several decade of pop culture relevancy. In the 50's Superman was a huge cultural phenomenon and Batman became one with the 60's Batmania.
These character have been more known and culturally relevant long before SW or even the more modern Superman and Batman movies. As far as Luke goes, he's barley in TFA and Vader is not even in it. SW is what sells...and they can put anyone like Rey and Kylo and SW fanboys will mindlessly buy it because of the old movies they grew up watching.
The only thing I'll give the MCU, is that I've seen certain guys...the jock type, wearing CA and Iron Man shirts. Still, those type of men would never wear a SW or ST shirt because the stigma is that's for nerds and geeks.