It’s fairly obvious that “Popstar: Never Stop Never Stopping” aspires to be a modern equivalent of “This Is Spinal Tap,” a mockumentary-style spoof of the contemporary music scene that’s, quite frankly, ridiculous enough on its own. It’s agreeable but overlong, and lacks edginess, coming across as more affectionate than cutting—a fact that’s amplified by the appearance of so many big names from the business in cameo roles (even Ringo Starr shows up as one of the interviewees, though Paul McCartney’s brief turn is just from file footage). The result is more a mildly amusing take-off on an episode of VH1’s “Behind the Music” than a genuine satire, but apart from an excess of raunchiness—something that has to be expected nowadays, especially in a movie for which Judd Apatow served as one of the producers—it’s inoffensive if bland.

In an expansion of the popular digital shorts that they made as adjuncts to Saturday Night Live, Andy Samberg, Jorma Taccone and Akiva Schaffer play Conner, Owen and Lawrence, whose hip-hop boy band Style Boyz soared to the top of the charts before the childhood friends had a falling-out. Now Conner is a solo star calling himself Conner4Real, while Owen has been relegated to the role of his onstage DJ. Lawrence, smarting over Conner’s lack of recognition for his contributions, has retreated to Colorado, where he’s become a simple farmer—though the state he’s chosen should be a giveaway as to what crop he’s growing.

The slender plot is the familiar rise-and-fall one that’s been a staple of music stories since time immemorial. Conner’s second solo album is a bust, sending his career into a tailspin. An ill-advised marketing deal with an appliance manufacturer makes him even more detested. His bumptious attempt at a socially-relevant single earns him nothing but scorn. His effort to bolster his tour by adding an ambitious rapper as his opening act goes wrong when the new hire proves a bigger draw than he is. A disastrous “wardrobe malfunction” makes him an international laughingstock. His beloved pet turtle falls ill (the obligatory vomit scene in this case is at least different, species-wise). And in the end he even offends his last true friend, long-suffering Owen, who at last abandons him.

Of course when Conner hits the skids, he’s ready for change—though like everything else in this movie, it’s pretty much skin-deep. After mending fences with Conner, Owen decides—as he puts it—to “Parent Trap” him, arranging a reconciliation with Lawrence that brings the old band together again for an astronomically successful reunion at an awards show. Conner has been redeemed, insofar as that’s possible with such a dim bulb.

“Popstar” inevitably plays more like a series of sketches than a real movie, and one suspects that they would have worked better as the individual digital shorts that inspired them than they do strung together into a feature. But while the picture does tend to grow a mite tedious as it runs on, its good-natured attitude keeps it from become insufferable, and there are enough decent gags and lines strewn throughout to elicit occasional smiles, if few belly-laughs—though the language is often rough in rap-star style. Samberg is hardly any great shakes as an actor, but he’s an amiable doofus, and both Taccone and Schaffer give him genial support. So does Tim Meadows, who gets some of the best dialogue as Conner’s worried manager, while Chris Redd brings some spice to Hunter the Hungry, the rapper brought on as Conner’s opening act. No one else gets much beyond cameo status, and some (Maya Rudolph as the appliance-company spokesperson) fare better than others (Will Arnett as a gossip-site bigwig, Joan Cusack as Conner’s freewheeling mother). The picture, shot by Brandon Trost and edited by Jamie Gross, Craig Alpert and Stacey Schroeder, exhibits more technical polish than you might expect (perhaps too much, given the material it’s spoofing), while the production design by Jon Billington and Sophie de Rakoff’s costumes catch the ambiance of the gaudy scene nicely.

While “Popstar” certainly chooses easy targets, it grabs the low-lying fruit to moderately amusing effect: the result is no “Spinal Tap” for today’s generation, but it’s certainly an improvement on the trio’s earlier effort, the awful “Hot Rod” (2007). Perhaps what typifies the movie most is the appearance of Justin Timberlake as Conner’s chef. The fact that the singer-aspiring actor would happily contribute to a movie that spoofs a guy very much like himself is a good indication that the picture is a genial take-off rather than a sharp satire. But for pop-music fans who know the performance-centered documentaries about people like Justin Bieber and One Direction, the one joke endlessly repeated may be enough to provide a reasonably good time.