Melissa McCarthy plays the title part in Tammy. Not too far in, she winds up standing on a kitchen countertop. By this point, a deer has totaled her car. She’s been fired from her fast-food job after showing up late and disheveled. Upon exiting, she tosses ketchup packets at her manager and rubs her bloody, sweating face all over a bag of hamburger buns. She comes home only to find her husband (Nat Faxon) making dinner for their neighbor (Toni Collette). Smelling the affair they’re having, she comes further unhinged.

Before he can tell her he’s leaving, she screams that she wants a divorce and runs into the kitchen, which has been framed in a wide shot so that McCarthy, still in the filthy fast-food uniform, is shown standing up on the counter while Faxon and Collette look up at her. It’s the only comedically wondrous image in the movie: McCarthy’s girth has a grubby majesty — a deep-fryer Falstaff. Half a minute later, she jumps to the floor, lands hard, and falls over, and the audience cracks up. That’s pretty much the joke: She’s fat and she’s nuts.

McCarthy produced and also wrote the script with her husband, Ben Falcone — who serves as the director, nominally. They give it a tired-but-manageable setup, but then promise a road movie for Tammy and her randy, alcoholic grandmother, Pearl (Susan Sarandon), which is right about where I detrained. The makeup artists have given Sarandon gray hair and intermittently swollen ankles. They might say they’ve tried to age her, but her agelessness keeps radiating through. It’s like passing off good wine as Kool-Aid.

Sarandon comes up with a stuttering gait. Still, however old this woman is supposed to be isn’t old enough. She and McCarthy spend most of the movie together, and you never believe the relationship. Even with the makeup, they could still be sisters. The film needs you to believe that Pearl is ancient and that her making out with a horny farmer (Gary Cole) in the back of her car is gross — and there might be a class of moviegoer that’s like Tammy, cringing at the sight of Pearl’s hand pressed against the rear window. Sarandon’s sensuousness just underscores the parade of incongruities. Allison Janney plays Tammy’s mother and Pearl’s daughter, which only exacerbates the casting confusion. By the time Kathy Bates steals the movie as Pearl’s sensible, straight-talking lesbian cousin, you don’t know what’s going on.

This raggedy project exists only because McCarthy opted to cash in some stardom chips. I don’t know what limits the studio placed on her and Falcone, but the entire movie seems preemptively safe. It’s probable that the only major risk is opening a movie on Fourth of July weekend with a female cast that averages about 52 years old. That’s also something to cheer. McCarthy is an actual bankable star, and she’s used her gathering clout to make a movie whose two funniest gags involve a slumped-over Sarandon.

McCarthy is in the Will Smith and Michael Bay slot but feels no need to dress for the occasion: She’s mostly in T-shirts, sweats, and Crocs. Her rise is more or less simultaneous with Kevin Hart’s, which implies that we’ve been missing what, for the movies, are comically built loudmouths. But their talents are being misused. With the exception of this winter’s About Last Night, Hart’s been running amok for four years. McCarthy gets marginally better material but needs a leash, too. But I think the studios are looking at the profit margins of their hits and thinking, Let them do what they want. What someone should do is find a reason to handcuff her to him. Anyone working on a Midnight Run remake: Now’s your chance.

In the meantime, we have Tammy. What hope should we have, though, for any road trip whose car gets wedged between two trees? There’s a mild stab at adventure that turns Tammy and Pearl into outlaws. Sarandon, of course, was half of one of the great movie road trips, and there are moments when Tammy seems poised to evoke it. But the bits of sex and crime don’t build to much, let alone something as anarchic as Thelma & Louise. There’s no screenwriting here, just loads of sketch comedy, like McCarthy’s attempt to rob a restaurant while wearing a takeout bag on her head. Maybe a dumb movie like this is possible because of Thelma & Louise. They went over the edge so women like McCarthy no longer have to.

But a female energy rules Tammy. Once Pearl and her granddaughter make it to Bates’s estate — where she and her girlfriend (Sandra Oh) throw an all-girl July 4 rager — you’re excited that McCarthy and Falcone will take the movie somewhere new and weird, that the rager will rock. Instead the ladies commit a Viking burial of a jet ski (don’t ask) while Tammy maintains a flirtation with that farmer’s son (Mark Duplass). This movie doesn’t know what it’s about. Neither does McCarthy’s comedy. She continues to do the equivalent of cannonballing into a pool: drenching us, laughing. But her “stupid blue-collar fat lady” routine is now a shtick. Sometimes it gets an ideal foil, like Kristen Wiig or Sandra Bullock; but on her own, you’re left to wonder whether McCarthy’s success is an extension of privilege. Could Queen Latifah or Octavia Spencer practice the same nutso buffoonery and be the box-office draw McCarthy’s become? They don’t really have the luxury to dare.

In that regard, McCarthy’s performance of unself-conscious uncouthness celebrates the freedom to be undignified. And, still, it’s being misused. The funniest line in Tammy is a throwaway. While pouring gasoline on a car (don’t ask, Part 2), McCarthy says, “Four dollars a gallon. Thanks, Obamacare.” The flavor of her delivery is Tin Pan Alley orphan. A good movie, a very smart one, could do wonders with that idiot persona. A great, heartless satirist — Armando Iannucci, say — should rope her into his or her circus.

In the meantime, we’re stuck with gutless sentimentality. At some point, the lunacy tends to break down, and McCarthy’s characters start to ask for forgiveness, to be loved. Her obnoxiousness is supposed to be tolerable because it has arisen from some wounded place. Identity Thief, another road-trip comedy, gave her an absurd last-minute sob story. This movie tries, too. McCarthy can do the blubbering required to sell pathology, but, ultimately, she doesn’t have anything truly psychological to play. Her violent blend of vulgarity, stupidity, and rotund physicality have made her a star, but it’s uneasy laughing. That body type is close to normal in the U.S. (Tammy showcases several full figures), but that’s not how McCarthy is letting the movies depict her body.

Over and over, McCarthy’s way into a joke in Tammy is through food — pies and Cheetos and an uncooperative vending machine — and brutal, almost narcissistic social awkwardness. There’s charisma driving all of her flopping around and bingeing and lashing out. But there’s also something deeper that these movies seem afraid to explore — a disorder. Tammy is told she needs to grow up; basically, to stop acting like Jerry Lewis, Will Ferrell, and Seth Rogen. That misses the problem. Why isn’t anyone telling her to see a shrink?