If Shakespeare was writing today, he’d be having a blast with “catfishing.”

The guy who wrote the book — the plays, actually — on “mistaken identity” romances, disguises, girls dressed as lads, etc., would have been all over social media’s creepy/funny/scary practice of pretending to be someone you’re not and the complications that ensue.

I’ll bet he could find a way to make catfishing romantic. Screenwriter Lindsey Beer, director Ian Samuels and star Shannon Purser? A bit beyond their grasp.

“Sierra Burgess is a Loser” is about the smartest girl in school, the daughter of a famous writer with chip-off-the-old-block (Alan Ruck) tendencies, Stanford ambitions and nothing at all that would get any guy’s attention at school.

Sierra (Purser) is freckled, plumpish and knows it. “You are a magnificent beast!” is her morning mirror affirmation, and hints at both confidence and self-awareness.

She has debate team, marching band, her probably gay BFF (RJ Cyler of “Me and Earl and the Dying Girl”) and the enduring contempt of the Mean Girl Three.

That would be Chrissy, Mackenzie and Queen Bee/Cheerleader Veronica (Giorgia Whigham, Alice Lee and Kristine Froseth).

Sierra has the thick skin to shrug off their insults, even Fairest of the Fair, Veronica’s. She has the brains to burn her right back.

“Move, before you break the mirror, Frodo,” in the bathroom gets a smart-girl correction.

“Quasimodo.”

Veronica’s revenge? Palming off Sierra’s number when some cute guy (Noah Centineo of “To All the Boys I’ve Loved Before”) who asks for hers. He texts and texts, and Sierra is overwhelmed, and then wised-up. She figures out what happened. But can she break free of this boy she’s connecting with? You know, tell him the truth?

“He’s imagining her when he’s talking to me…but…they’re MY words!”

As Mean Veronica is a bit of a dumb blonde, jilted by some college freshman she’s dated, Sierra strikes a bargain. Pretend to be interested in the guy who wanted your number and Sierra will “teach you to study.”

Sierra makes a shocking discovery. Veronica’s home life is a comic fallopian hell — shrieking little sisters, a pushy mom living vicariously through her. Veronica’s “discovery?” Sierra is smart, and smart can be cool.

“‘To be or not to be,’ I’ll have to teach you that next.”

“Nietzsche is like a sexy German vampire. He whines a lot and thinks everything’s pointless.”

BFF Dan frets at what could become of Sierra’s new occupation with boys — “Things escalate, texts turn to calls, calls lead to hand holding, holding hands lead to teen pregnancy, unemployment, lady baldness.”

Purser, of TV’s “Stranger Things” and “Riverdale,” doesn’t have funny in her bones. She makes Sierra thick skinned but not needy or touching enough to root for. Her line readings are teen-real — rushed blurts — which prevent her jokes from landing, ruining her few funny lines.

Even Cyler seems a bit off his wacky-sidekick game here — “Are you a catfish, or a ‘can’t fish?”

They’re not helped by the production. Their wittiest exchange comes in the middle of band practice, blurted out between flute or clarinet parts in the arrangement that’s being played around them. Hilarious? No, mostly inaudible, killing the timing.

Froseth has a winsome way with the prettiest girl, the “Dorian Gray” of high school, suffering in her own way and compensating by lashing out. Even when she’s not lashing out, her little kindnesses have a tactless edge.

“She’s not a lesbian. She just has no taste.”

“Loser” has flashes of empathy and a high mindedness about literature and philosophy and book learning in general.

It’s just that it’s light on “heart,” and has a touch of what the Bard labeled “lackwit” in its banter and the ways its few funny lines are played.

Screenwriter Beer kicks the most interesting relationship, Sierra and Dan, to the side. The two smartest kids in school palling around, rivals in seeking something outstanding or just odd to make them “stand out” in their college applications.

Little grace notes are scattered around the edges — a girl who turns a poetry assignment into a furious rap, the hapless track coach (Geoff Stults) forced to let resume-padding lumps try out for his team — don’t overcome the general sourness of the proceedings.

Slapping a bunch of teen comedy stars of yesteryear in supporting roles — Ferris Bueller’s pal Ruck plays Sierra’s dad, Lea Thompson is her mom — doesn’t bestow that John Hughes magic on the film. Giving the hilarious, empathetic Loretta Devine nothing funny to play (she’s the English teacher) is criminal.

So yes, “Sierra Burgess IS a Loser.”

MPAA Rating:PG-13 for sexual references, language, teen partying and thematic material

Cast: Shannon Purser, Kristine Froseth, RJ Cyler, Noah Centineo, Loretta Devine, Giorgia Whigham, Alice Lee, Alan Ruck

Credits:Directed by Ian Samuels script by Lindsey Beer. A Netflix release.

Running time: 1:45