Garry Marshall continues to plunder holidays for ensemble dramedies that resemble nothing more than extended episodes of the old “Love, American Style” TV series. Having polished off “Valentine’s Day” and “New Year’s Day,” he now turns to the Atlanta-set “Mother’s Day,” and comes up with a picture that continues the downward trajectory of the uninspired series. One shudders at the thought of how terrible further installments might be.

The lion’s share of the running-time is devoted to Sandy (Jennifer Aniston, working her usual flustered shtick to the maximum), a divorced mother of two adorable young sons whose cool-as-a-cucumber ex (Timothy Olyphant) abruptly announces his marriage to a much younger woman (Shay Mitchell). Sandy’s appalled not only by his nonchalance but by their new stepmom’s attempts to connect with the boys. She’s also professionally frazzled, especially when she tries to land a designing job with Miranda (Julia Roberts, looking like a grimacing statue topped by a hideous wig), the imperious head of home shopping network apparently specializing in “mood pendants.” Miranda’s agent is played by Marshall regular—and supposed lucky charm—Hector Elizondo.

Sandy’s the link with several other plot threads. She has just joined a fitness center run by Bradley (Jason Sudeikis, bland), a widower still mourning the death of his military wife (Jennifer Garner, seen briefly in a clip singing karaoke while on her last tour of duty) while trying to raise their two daughters on his own, and becoming a helicopter dad in the process. And she’s buddies with Jesse (Kate Hudson), who’s married to Russell (Aasif Mandvi), with whom she has a young son, and Jesse’s younger sister Gabi (Sarah Chalke), who lives next door with her partner, a woman named Max (Cameron Esposito) and their adopted son. The sisters’ lives will be thrown into chaos when their redneck parents Flo and Earl (Margo Martindale and Robert Pine) arrive unexpectedly from Texas in their RV, totally offended that one of their son-in-laws is Indian and the other a female.

As if all that weren’t enough, we’re also introduced to Kristin (colorless Britt Robinson), a waitress who works alongside her boyfriend Zack (Jack Whitehall, enthusiastic but exhausting), who tends bar while taking unpaid gigs as a would-be standup comic (a career goal that, on the basis of what we see of his miserable act here, seems utterly misguided). They have a child, but Kristin resists marrying because she remains emotionally shaken by the fact that she was given up for adoption. But she’s tracked her biological mother down. Guess who she is.

Given the small army of characters stuffed into the screenplay, it’s not surprising that none of them emerges as more than a sketch, some of them the crudest sort of stereotypes (Flo and Earl are certainly an insult to all Texans). But rest assured that all the problems they face will be resolved with remarkable ease, in typical sitcom fashion. (Flo’s bigotry, for example, is washed away by a Skype conversation with Jesse’s Indian mother-in-law, played by Anoush NaVart, and a brief gambol with her grandson.) The humor on display is of sitcom quality too—though that description is more than a little unfair, since the writing in some sitcoms is of pretty high quality (remember Marshall’s version of “The Odd Couple”?), and that here is definitely not. The laughs are scarce, and some of the gags are gruesomely unfunny in that “naughty” way familiar from crummy “family” pictures. Occasionally an in-joke is attempted, with disastrous results. (When Sandy describes a humiliating moment she arranges for her ex as “justified,” for example, it’s difficult to suppress a groan.)

The craft team, headed by cinematographer Charles Minsky, have given “Mother’s Day” a fairly attractive sheen, though the work of editors Bruce Green and Robert Malina seems rather sluggish (the movie runs, unconscionably, for nearly two hours, though that’s mostly due to Marshall’s lackadaisical pacing) and John Debney’s tinkling score is almost as constant an irritant as the trite dialogue. While the surface is nice enough, though, the substandard content means that the movie is more like “Trash Day.” If you have some sort of grudge against your mother, however, it might prove a suitable destination for an outing.