A grubby “Forrest Gump” minus the computer-generated presidents and the charm, David Spade’s lowbrow new film traces the efforts of the title character, a back-home hayseed, to track down the parents who abandoned him at the Grand Canyon when he was just a tyke. By painting its hero as a bumbling naif with good intentions despite his sloppiness and ineptitude, “Joe Dirt” wants you to feel a mushy heart beating beneath its surface of slapstick vulgarity; but neither the farcical veneer nor the underlying would-be pathos works. The episodic result has a few funny moments, but mostly it feels like a succession of pretty bad sketches from one of the less noteworthy seasons of Saturday Night Live.

The comparison to Spade’s old late-night gig is emphasized by the fact that the story is told as a series of flashbacks as Joe is interviewed by a smirking radio talk-show host, played by Dennis Miller with the same attitude of snide smugness he brought to “Weekend Update.” Returning periodically to the present for another dose of Miller-Spade banter (which unfortunately comes across as dead air), the movie–after explaining Joe’s peculiar hairdo with an especially gross anecdote–runs through the poor kid’s solitary life (the sole bright spot being his friendship with a sweet young thing named Brandy, played by Brittany Daniel) before he undertakes his comic quest for his family, and then regales us for eighty minutes or so with jokes involving such hilarious items as incest, flatulence, the sexual proclivities of dogs, dismemberment, nuclear explosions, mob hits, food fights, vomit and excrement before settling into a heartwarming conclusion wherein Joe, now a media celebrity, finds not only his trailer-trash mom and dad but, more importantly, true love.

Throughout the string of misadventures Joe is perpetually optimistic, even as fate deals him one stinging blow after another. The poor sap’s explanation for his cheerful attitude, as he occasionally tells some onlooker, is that “Life is a garden–you have to dig it.” This sounds a lot like Gump’s famous dictum about life being like a box of chocolates, but in reality it’s crudely lifted from a much better picture–Hal Ashby’s wonderful 1979 adaptation of Jerzy Kosinski’s “Being There,” in which Peter Sellers, as the blank-slate Chauncy Gardner, astounded his audiences with a similarly empty pronouncement. Suffice it to say that Spade, with his nasal voice and inability to draw a character beyond a cardboard exterior, is no Sellers, and that feature neophyte Dennie Gordon is no Hal Ashby.

There are a few funny moments in “Joe Dirt,” though. Most are provided by Christopher Walken, who brings his usual air of bemused oddity to the role of a kindly school janitor with a secret. (The pseudonym he adopts at the end is a great in-joke.) One can also enjoy Adam Beach, whose deadpan niceness as an Indian who runs a fireworks stand is given too much screen time but is a pleasant change from the aggressive behavior displayed by most of the other characters. (The latter group includes Kid Rock, who demonstrates no more talent as Joe’s rival for Brandy’s affections than he does as a singer.) There’s also a hysterical thirty seconds or so when our hero encounters a New Orleans passerby (Blake Clark) with an impenetrable Cajun accent. One presumes that Rob Schneider was unavailable for the cameo.

But the moments of amusement are fleeting and far-between. “Joe Dirt” is the sort of fractured slapstick farce that will please only the most undemanding of audiences, and even they would be well advised to wait the few months it will take for it to hit the video bins.