In Stranger Things, Nancy is the "pretty girl." She's set up in a kind of 80s teen movie protagonist role, both by Eleven and by the show itself. We meet her as the main boy's older sister, and she ticks off so many tropes that we probably think we can predict where her story is going. She's the girl who wants to be popular, with a jerk boyfriend, disagreements with her mom, and an awkward nerd guy waiting in the wings who she's obviously destined to be with as a reward for his inevitable heroism.
She's also the girl that Eleven borrows dresses and make-up from, the sort of girl that Eleven is apparently trying to emulate -- and delighted about emulating -- when the boys stick a wig on her and try and make her look "normal" for school.
And this "pretty" plotline seemed to annoy a lot of people. Eleven is, after all, simultaneously an extremely traumatized child and a paranormal-powered badass. We meet her as the polar opposite to Nancy --the supernatural experiment girl with a shaved head who grew up in a lab, and has always been used for other people's ends. She doesn't know words like "friends" and "promise," but she does know "pretty," and she seems to treasure the idea that it could ever apply to her.
But just as "friends" and "promise" take on great meaning over the course of the show, "pretty" to Eleven doesn't really seem to mean "pretty." It's being "normal," and, with it, being worthwhile.
Assumedly, Eleven knows about "pretty," but not about "friendship" and "promise," because friendship and promises are things that she would benefit from, and so not necessary for a science experiment to know. "Pretty," on the other hand, is a concept of the self that exists for others, and so is probably something she's encountered, along with the idea that girls are supposed to be pretty, and that having a shaved head and controlling things with your mind doesn't fit the bill. It's her desire to be live a normal life and to be accepted, expressed in the only way she knows how.
But of course, her initial version of "pretty" is an 80s middle school boy's idea of what a girl looks like, and is miles from "pretty girl" Nancy's reality. Because sure, Nancy might be "pretty," but she's not existing in a teen movie here. She's a total badass. An actual, fearless badass, who fights beasts and crawls through portals in trees to find her friend, who cuts her own hand to draw the murderous monster to her, who sets traps for supernatural beasts and sets them on fire. She'll turn a gun on her boyfriend to get him to leave when the monster is bearing down on them, and she won't even flinch. She's fierce as anything in searching for her friend, protecting her brother, and general monster-hunting awesomeness.
She's set up with that "pretty girl teen movie" story, and she repels it at every turn. When her mother tries to make their discussion about her secret boyfriend and not her possibly dead friend, Nancy reacts in disbelief. When Jonathan tries to pull the "I see the real you, I know you're different" cliche nonsense, she shuts it down at once. She has some of those teen movie sideplots, but she has far more important things to deal with.
At first glance, Nancy and Eleven are as different as its possible to be. But the introduction of Nancy's pink dress creates an initial superficial comparison between them -- which is interesting, because we never see Nancy wear the "pretty girl" dress herself, in part because she's not this idea of cliched normality that the phrase invokes. And in fact, the girls have a lot in common -- they're both fighters, although Nancy is the more physical of the pair. They're both driven by friendship and a desire to help and protect others, and they're both brave and self-sacrificing, although Eleven is self-sacrificing when there's no hope, and Nancy puts herself at risk to create hope.
If pretty is a stand-in for normality for Eleven, then Stranger Things is the story of how she moves beyond that. First, she says it when she's transformed into a cliched idea of girlhood -- a picture of what we might originally expect Nancy to be too. But that isn't Nancy, and it never was, whether she's pretty or not. We next hear the idea when Eleven is looking at her real self in the mirror, and she says "still pretty," as she starts to accept herself and see the value in her uniqueness. Nancy is still pretty, or worthwhile, when covered in gore, and Eleven is still worthwhile as herself. And finally, the idea of "prettiness" fades behind more important concepts she's learned about, the concepts that fuel all the characters -- friendship, promises, and, most of all, bravery. Because what's being "pretty" compared to all of that?