Tina Fey enjoys the career every sharp-witted writer-performer in America, female or male, dreams of having. She's a paragon of nerd-babe wit and wisdom, sharp-edged but with an appealing undercurrent of reticence. (Part of her always seems to want to get behind the camera and watch someone else do whatever scene she's doing.) The Second City alum headlines a self-created hit sitcom ("30 Rock"); she made an impact on national politics with her recent, exquisitely timed morale boost to Hillary Clinton supporters across the nation; she graces magazine cover after cover; and she hawks for American Express. She's everywhere, and she's good at every one of her jobs.

The new film "Baby Mama" showcases Fey opposite a fellow "Saturday Night Live"-trained comic actress, Amy Poehler. The prospect of seeing a mainstream commercial comedy built around these two sounds tasty enough. Their stylistic differences—Fey's a chipper ironist, Poehler's a saucer-eyed caricaturist—hold promise as well. And for a while, despite the hacky '70s-sitcom technique behind the camera and in the editing room, I went with it. The performers include genial Greg Kinnear (who plays Fey's love interest) and Steve Martin, who plays Fey's eccentric, pony-tailed boss. The way Martin utters the phrase "amazing salmon," well, it's very nearly nine bucks' worth.

Midway through, though, I started wondering why I wasn't laughing more. "Baby Mama" was not written by Fey and/or Poehler, which may be the reason. The film was written and directed by Fey's former "SNL" cohort Michael McCullers, who may well be a jokesmith, but retorts such as "Where did you read that—the Weekly World Dum-Dum?" barely qualify as jokes. As a first-time director he has yet to develop a whisper of visual style or strategy. Fey and Poehler no doubt had a hand in shaping the material, and they're as engaged as possible. But baby formula is baby formula.

Unable to conceive on her own despite copious Post-It notes to herself ("Yes! Be fertile!"), Fey's Kate Holbrook hooks up with a pricey surrogacy center run by an imperious blueblood (Sigourney Weaver). One hundred thousand dollars later, Kate scores her "baby mama" in Angie (Poehler), pure white trash with an ill-defined New Age streak (talk about moldy jokes) and a common-law husband played by Dax Shepard. The Kate/Angie relationship develops as one-half Felix and Oscar, one-half Thelma and Louise. Kate patronizes the working-class lout Angie endlessly, but you know the latest reconciliation is around the corner. One minute Kate and Angie are at each other's throats; the next, they're doing karaoke and bonding.

Kate's an executive at a Whole Foods-type organic foods grocery chain. This is a milieu begging to be satirized. So is surrogacy, though certain details in "Baby Mama" ring false. Kate's panicked about being the "oldest mom at the preschool." At a measly 37?

Every moment of this project feels beat-driven, focus-grouped and designed to package Fey as a viable movie star with great pins (as one character takes pains to note) to go with the breasts (ditto). This isn't writing, it's advertising. Poehler's character is a random collection of disparate quirks, and while Poehler has her moments—I like the way she pretends to look through her pockets for gas money—her energy is all about sketch-comedy attack. The film probably will find a wide audience, because people like Fey, and it's going for likable rather than side-splitting. Still, a line from "Mean Girls" (which Fey wrote) kept nagging at me. "You don't have to dumb yourself down to get guys to like you," Fey reminded Lindsay Lohan. Substitute "a mass-market film audience" for "guys," and you have an apt reminder for Fey and her collaborators.

See the trailer and find local showtimes for "Baby Mama."