Mila Docheva faced down the camera, her contours camouflaged by an outsize hoodie raucously inscribed with neon graffiti. Her turnout, which she posted on Instagram, was a homage to her idol, the pop star Billie Eilish.

“When I dress like Billie, I feel just a little bit closer to her,” Mila, 14, said through a translator, in an email (she lives in Silistra, a town in northeastern Bulgaria). “I’m not a girlie girl. I am curvy, and I ride a skateboard. Feeling comfortable in my skin is important to me. Billie’s style gives me the courage to do that.”

Swarms of girls and young women ranging in age from under 10 to their early 20s are echoing those sentiments. They are smitten with Ms. Eilish, who took home an armful of Grammys this week, remodeling themselves in her image, posting gaudy makeovers on Instagram and other social platforms, from places as far-flung as Tbilisi and Nashville.

Ms. Eilish’s following has swelled since her first single, “Ocean Eyes,” went viral on SoundCloud five years ago. Her style clout has grown alongside it, on infectious display on Instagram, where she has millions of followers. It recalls the kind of fashion fervor rarely seen since Madonna’s early videos incited throngs of very young fans to shimmy their frames into merry widows and pile on stacks of bangles and cataracts of chains.

The enthusiasm of Ms. Eilish’s devotees denotes a striking turnabout, a new generation’s rejection of the flirty babe aesthetic embodied by contemporary idols like Ariana Grande in favor of something more crazily improvised and less strenuously sexual.

At 18, Ms. Eilish, who often goes without makeup, favors a pastiche of outsize 1980s and ’90s hip-hop and skater looks. That look speaks assertively to a Gen Z crowd chary of artifice and aggressive displays of sensuality.

“Her look is not about vanity,” said Lucie Greene, a trend forecaster and brand strategist. “She is flipping the idea of beauty to something surreal, something influenced by gaming and the cyberculture.”

“These are not the filtered images of millennials,” Ms. Greene said.

Since arriving on the pop scene, Ms. Eilish has underplayed, and even willfully sabotaged, the vampiness that has long been deemed a prerequisite to stardom. To some she is the anti-Ariana, a tomboyish naïf.

Her look, paradoxically standoffish and in your face, is punctuated by shocks of blue or green hair, riotously patterned tops and tracksuits, and thorny punk and goth references. It is, in Mila’s phrase, “an act of rebellion,” as provocative as it is protective.

“She has an armor that’s bright and loud,” said Rachel Gilman, a fashion stylist whose clients have included Bloomingdale’s and Adidas. “It’s dye your roots green and wear the baggiest clothes in the room. It’s good for girls to see that they can succeed without wearing a push-up bra if that isn’t their vibe.”

The star’s advent, some say, could not have been more timely. “It feels fair to describe Eilish’s aesthetic as both an antidote to and a queering of the hyper-feminized pop-star archetype,” Amanda Petrusich wrote in The New Yorker last summer. Her style resonates, Ms. Petrusich said, “in a cultural moment when we are all trying very hard to sort out real people from the ones who are merely savvy and ambitious enough to know the right way to curate and present an authentic-seeming vibe.”

Ms. Eilish’s apparent realness has inevitably snared the attention of marketers as well. Her appearance last spring in the “I Speak My Truth in #MyCalvins” ad campaign contrasted sharply with previous Calvin Klein campaigns modeled by scantily clad boys and girls.

Only last month H&M sought to cash in on the impact of her socially conscious, street-inflected message, releasing a line of sustainably produced sweatshirts, supersize hoodies, bucket hats and beanies.

Ms. Eilish presents as audacious and spontaneous, if a bit candied at times. “She dresses like a Disney princess, but she doesn’t sound like one,” said Giana, a 10-year-old social media personality from Dallas with a shock of emerald-tone hair and an Instagram following of her own. To Giana, who goes by her first name only and dresses in Eilish-inspired clashing plaids and colors, Ms. Eilish is the mistress of brash self-invention.

“She knows you can make a look fearless,” Giana said, “and she makes me feel it too.”

It may jolt her to learn that Ms. Eilish once scorned wannabes, venting that resentment in her lyrics. “Copycat trying to cop my manner/ Watch your back when you can’t watch mine,” she taunts in “Copycat,” released in 2017.

She has certainly vaunted her originality. “I could easily just be like, you know what, you’re going to pick out my clothes, someone else will come up with my video treatments, someone else will direct them and I won’t have anything to do with them,” she said in a profile in The New York Times last year.

“But I’m not that kind of person and I’m not that kind of artist,” she said.

She seems happy enough, all the same, to exploit an ever-widening fascination with her fashion bravura, entering into partnerships with Freak City, an online platform based in Los Angeles, and with youth-oriented stores like Urban Outfitters and Hot Topic. She has introduced a children’s line on Blohsh, the Billie Eilish online store that sells vinyl albums and CDs, along with fleece blankets, socks and sweatshirts with spray-painted logos.

Some fans may chafe to discover that she has worked since she was 14 with Samantha Burkhart, a high-powered stylist whose clients include Katy Perry, Christina Aguilera and Mark Ronson. Her outfits are excessive, Ms. Burkhart said.

“She enjoys the uncomfortableness of not fitting in,” she said. “That appeals to a generation that grew up sick of manufactured pop music produced in a way that follows algorithms.”

Attuned to her charge’s extravagantly whimsical outlook, Ms. Burkhart insisted that she doesn’t impose a look but simply functions as a kind of a personal shopper, gleaning items from the marketplace, which Ms. Eilish puts together herself.

The devout seem unfazed either way. “In most of her songs she talks about being alone and afraid,” said Alisa Gordon, 18, a college student and aspiring actress in Nashville. “When I’m watching her music videos, everything makes more sense. It all comes together like a puzzle.”

Some fans react with a more diffuse anxiety, the sense of a world in peril. “Why not respond to news that the planet is warming to uninhabitable levels by matching your hair to your parka to your nails to your sunglasses to your shoes and socks,” Ms. Petrusich wrote, not altogether facetiously.

But admirers perceive a more earnest intention underpinning Ms. Eilish’s over-the-top appeal. “She makes ethical choices in her everyday life, from wearing ethically produced fashion to leading a vegan lifestyle,” Mila said. “Many people don’t take outspoken young girls such as Billie and Greta Thunberg seriously. They diminish them as being just children. But I’m grateful to have them as role models.”

There are times, of course, when a look is just a look. It can be a plea for attention — or signal a healthy self-regard. Karin Ann Trabelssie, a 17-year-old high school student from Jelina, in Slovakia, feels an emotional and aesthetic solidarity with the star. Like Ms. Eilish, she has long sought refuge inside baggy shirts and hoodies, but until recently she tended toward the dark or nondescript.

“I was bullied,” Karin said. “People tried to discredit me for being young, or sometimes just being a girl.” In recent months she ditched her somber camouflage in favor of brightly colored tracksuits and matching shoes.

“I used to be anxious about people looking at me,” she said. “Now I realize when they stare, it doesn’t have to be in a bad way.”

Jessica Cropp, an 18-year-old high school student in Verona, N.D., has adopted a more consciously stylized appearance since discovering her idol, and now peacocks on her Instagram feed in blue hair, shrill hues and many-layered silver chains. On occasion black tears course down Ms. Cropp’s cheeks, a spooky reference to the video for “When the Party’s Over,” in which Ms. Eilish swallows a murky liquid and weeps ink.

Ms. Cropp herself has not gone full goth. “Wearing bold colors boosts my confidence,” she said. “Now when I walk into school in the morning, it’s like everyone is going to look at me, and I’m going to love it. It’s like, I want those eyes on me.”