The artistry! The emotional range! The sheer diversity of human experience captured on celluloid! Consider these highlights from the cinematic oeuvre of Jessica Simpson: In The Dukes of Hazzard (2005), playing the big-screen incarnation of Catherine Bach’s Daisy Duke, a scantily clad Simpson bends under the hood of a Jeep, wiggling her butt in the air and cooing to a police officer, “I think something bounced up into my undercarriage.” In Employee of the Month (2006), as the beautiful new cashier at a Sam’s Club—style superstore, Simpson tenderly confesses to co-star Dane Cook that she’s not quite as perfect as he thinks—and then pulls back her hair to reveal a set of outsized ears. And let’s not forget the barely released Blonde Ambition (2007), starring Simpson as an “Okie bimbo” who nearly causes an international incident when she accidentally tells a group of Norwegian priests that their women “smell like fish.” Worry not, though, for our heroine: She saves the day by taking the priests to a karaoke bar, where they spiritedly perform a rendition of Sir Mix-A-Lot’s “Baby Got Back.”

Nine years after she first burst onto the pop charts as a slightly more wholesome (and considerably more plastic-sounding) alternative to Britney Spears and Christina Aguilera, and five years after she rose to superstardom on the strength of her MTV reality show, Newlyweds, the time has come to write the obituary for Jessica Simpson’s fifteen minutes of fame. This isn’t a task one should approach gleefully. Indeed, for at least a few weeks during the sublime first season of Newlyweds, in which the Abilene-born performer mispronounced one word after another, much to the consternation of her then husband, Nick Lachey, it seemed as if Simpson might prove to be more than another blond Los Angeles celebutante. Her goofy double takes and Barbie doll earnestness hinted at a kind of Lucille Ball for the iPhone set: an adorable, Texas-accented airhead much too busy text-messaging her BFFs to worry about the fact that she’d just referred to a platypus as a “platymapus.”

Except Simpson’s subsequent film work hasn’t exactly got Kate Winslet and Natalie Portman looking over their shoulders. Her fourth feature, the Private Benjamin—inspired Major Movie Star, about a prima donna actress who goes to boot camp, was supposed to arrive in theaters earlier this year, but it’s rumored that the film’s backers are so dismayed by the finished product that they will send it straight to DVD. These days, Simpson, who divorced Lachey in 2005, is struggling to reignite her music career (the single from her first country album, which debuted in early September, has gotten mixed notices). But she seems much more engaged with the dubious art of sustaining her own fame, as evidenced by a string of high-profile romances (Dane Cook, Adam Levine, John Mayer, Tony Romo) that have kept the paparazzi in hot pursuit long after the rest of us stopped paying attention. My prediction: Give her two more years, and we’ll see her reality-TV career arc come full circle, with a “comeback” program along the lines of Denise Richards: It’s Complicated or Pamela Anderson’s Pam: Girl on the Loose.

How does a promising talent like Simpson go so awry so quickly? You can hardly blame her for leaping at the high-profile opportunity to fill Daisy Duke’s boots. But didn’t Simpson and her father/manager/career Svengali, Joe Simpson (who is credited as a producer on Employee of the Month, Blonde Ambition, and Major Movie Star), realize it was a bad idea to have her play the same seemingly-dumb-but-secretly-brilliant ingenue in each of her movies? Don’t they understand there’s a fine line between satirizing small-town, middle-American life and obnoxiously condescending to it? And that both Blonde Ambition (in which Simpson’s rube confuses a “tapas bar” with a “topless bar”) and Employee of the Month (which makes minimum-wage work look positively heavenly) are on the wrong side of that line? Even Simpson’s striking physical appearance—that helmet of flaxen hair, those gleaming red lips, those impossibly white teeth—has been tainted by her run-amok narcissism. (The Dukes of Hazzard, Employee of the Month, and Blonde Ambition all feature scenes in which the actress struts into a room in slow motion, boobs proudly pitched forward and long hair flowing behind her, as the male characters’ jaws go sinking to the floor.)

Simpson’s biggest problem, however, is at once very obvious and poignantly complex: She’s a comedienne who’s lost her straight man. A pouty-lipped slab of all-American frat-boy beef who, much like his ex-wife, toiled in the margins of the Hollywood B-list before marrying, Lachey wouldn’t seem to hold the key to Simpson’s success. But take a closer look at the often inspired Newlyweds, which ran for three seasons on MTV and chronicled the fledgling marriage of these two fame-hungry souls. What made the show so funny was Simpson’s increasingly baffling stream of profundities (Chicken of the Sea is not made of chicken! Buffalo wings are not made of buffalo!). But what made the show so entrancing were the flashes of contempt we often caught in Lachey’s eyes; he was the guy who had landed the fantasy wife only to come down with a crippling case of buyer’s remorse. The result proved unexpectedly complicated for a program that aired alongside the likes of Pimp My Ride and Date My Mom: a wicked evisceration of the emptiness of modern celebrity life that nonetheless adored the celebrities it chronicled.

Simpson without Lachey is Charo without The Love Boat. Separated from the unholy union that made her distinct, she now seems merely crass, commonplace, even a little desperate. Indeed, this has been her biggest mistake. She assumed she was America’s sweetheart, when in truth America had a much more nuanced, love-to-hate/hate-to-love relationship with her. And instead of pursuing film roles that might have traded on these mixed feelings of ours—the way, for instance, Scarlett Johansson has carved out a curious niche for herself playing naïflike seductresses in movies like Match Point and The Other Boleyn Girl—Simpson tiresomely keeps trying to fashion herself into a modern-day Goldie Hawn or Melanie Griffith.

Maybe she could put herself back on track by finding a role that reveals a side of her we’ve never seen before, in a big-screen musical comedy like Mamma Mia! or Hairspray. But don’t count on it. Without Lachey around to be exasperated by her—to play the slightly meaner version of Ricky Ricardo to her slightly more inane version of Lucy—Simpson’s dim-wittedness is just a joke in search of a punch line. And once a celebrity stops having control over her own joke—once she becomes fodder for others’ jokes, as happened when she was accused of being a “Yoko Romo,” whose distracting presence caused the Dallas Cowboys to lose a 2007 game against the Philadelphia Eagles—she’s officially one minute away from oblivion. And counting.