Rebel Wilson’s niche in the cinema is an enviable one.

She’s the quintessential “funnier best friend” the ultimate icing on the ensemble comedy (or musical) cake. The Aussie has made a career out of sharing the screen in “Pitch Perfect” or “How to Be Single” or “Bachelorette” — packing a lot of comic value into her moments and stealing virtually every scene she turns up in.

But being a leading lady isn’t about showing up on set and riffing your way to the funniest take, doing that for a few days or weeks, and moving on. It involves a lot more heavy lifting, a lot more range and in the case of “Isn’t it Romantic,” providing all the laughs when the supporting players aren’t in the Rebel Wilson league.

And if this rom-com that’s a send up of romantic comedy conventions and cliches and doesn’t definitively show she’s not up to the demands, it underlines how even Rebel Wilson can’t save a script this joke-starved or direction this uninspired.

Still, we could see why this was pitched, financed and filmed with every trailer for it. The idea, a rom-com that sends up rom-coms, is a winner.

Wilson plays a woman who was always taught that romantic comedies are fables, that as her mother (Jennifer Saunders) put it, “there is no happy ending” for women “like us.”

As in not svelte, beautiful with perfect hair and makeup from the moment they wake up like the star of virtually every romantic comedy ever.

Adulthood reinforced that. Natalie might be a Manhattan architect, but the plump pushover from Oz is treated like the office doormat, even by underlings. Only her adoring colleague Josh (Adam Devine, still not funny) and assistant Whitney (Betty Gilpin of “GLOW”) support her.

And Whitney’s a bit of a dope, endlessly watching rom-coms like “The Wedding Singer” on her work PC. Natalie cannot set her straight often enough.

“All these movies are terrible lies set to pop songs!”

Natalie’s just proven how “invisible” she is to men like their “He’s CW hot!” new client (Liam Hemsworth) when she takes a blow to the head during an NYC subway mugging on the way home. As she wakes up in hospital, something isn’t right. The doctor’s a contender to be the new “McDreamy.”And the decor…

“This isn’t an emergency room! This is a Williams SONOMA!”

She’s dressed in “Pretty Woman” (post-makeover) wear, sent out into the street, where seagulls fly in heart-shaped flocks and “New York doesn’t smell like s–t any more!”

Much bleeped profanity later, Natalie figures it out.

“I”m trapped in a f—–g PG-13 romantic comedy!”

She rages at her cliched interior monologues, is slack-jawed at the over-the-top interest Mr. “CW hot” shows in her (He writes his phone number on a rose. Don’t try this at home.) and sort of loses it at all the insipid pop that underscores every “Thousand Miles” step she takes on these fantasy-New York streets, into her now luxurious apartment and its over-stocked walk-in closet and makeup vanity.

Her testy neighbor (Brandon Scott Jones) has been turned into “an offensive version of a gay guy,” offensive and funny. Before she knows it,”I think we’re being dragged into some dumb ‘makeover montage.'”

And on and on it goes. First we identify the exhausted cliches of romantic comedies — gay BFF, every woman is a cutthroat rival (Priyanka Chopra takes after Adam Devine’s Josh), every New York streetside shop sells cupcakes, flowers or wedding gowns and when the right song pops up — random strangers all work with the same choreographer. Then we try to make something amusing out of sending up those cliches. And fail.

A “My Best Friend’s Wedding” gag repeated in too many rom-coms to number almost works — “karaoke night.” But just when you think “That’s it. ‘Pitch Perfect’ Rebel just needs a few more production numbers,” the movie provides them. They don’t help.

Hemsworth turns out to be the less funny sibling to brother Chris, although, truth be told, nobody here gets much of a break from the trio of screenwriters.

Waiting around for that first giggle is like sitting through a teetotalers’ Irish wake — death itself. And Wilson, letting the strain show the way a hundred other funny folks expected to “save” a flimsy comedy built around them have before her, has never seemed more out of her depth.

Here, she’s a Rebel without a laugh.

MPAA Rating: PG-13 for language, some sexual material, and a brief drug reference

Cast: Rebel Wilson, Liam Hemsworth, Adam Devine, Priyanka Chopra, Betty Gilpin and Jennifer Sanders

Credits: Directed by Todd Strauss-Schulson script by Erin Cardillo, Dana Fox and Katie Silberman. A Warners/New Line release.

Running time: 1:28