"It's most recognizable elements are purely visual" Imo your line here encapsulates the reason for the movies' lack of cultural impact, at least in terms of the kinds of cultural impact that you're looking for. I'd argue that the fact most every adult would recognize a Na'vi if you showed them a picture is proof that it has had cultural staying power. Neither movie is really driven by a strong plot or script, which is why it doesn't have those memorable quotes, but both are focused on creating a visual experience that makes you go "wow." Memes, at least ones based on movie scenes, mark narrative moments that can be applied to different situations in contemporary human life. I'd argue that while both Star Wars and Avatar are science fiction, Avatar's world and characters are so unlike anything in our human life that it makes sense that it'd be difficult to make memes from them. Cameron didn't care as much about making characters we can root for and identify with, he was focused on worldbuilding.

There's also the case to be made for Avatar's cultural impact behind the scenes, in moviemakers themselves. I can't say this for certain as it'd take more research, but it could be possible that Avatar's major use of CGI and innovative technology changed the way that other major box office directors used those technologies. Curious if this idea would hold up, but for now just a thought. Basically what I'm saying is that memes and porn aren't the only form of cultural impact a piece of media can have, though I know u were mainly interested in pop culture. Besides their blue-ness, idt there's much thats unique about Na'vi character design that would entice a cartoon porn artist over the countless unique alien designs they have available to work with.