How do you define mainstream? Sure it has to vary with demographics etc. But if you ask any random person, "have you ever heard about Star Trek?" , you will get a yes for an answer in too many cases, even if they've never watched it. Show them images of the characters and they'll likely go "aahhh yes those". Compare that to something more obscure like "have you ever heard about Aeon Flux?" and you'll probably get weird looks even though there are comics, a movie and an animated series of it. Star Trek may not be as popular as other franchises, but it is mainstream imo.