Some of Hollywood’s most durable genre conventions have to do with outsiders and underdogs, often two categories rolled into one, who show up the self-important elites. The cowboy who rolls into town and brings justice in a not-quite-law-abiding way. The lovable con artist who makes a fool of the uppity society folks. The washed-up cop or spy called in for one last covert mission. The stereotypical sorority girl who turns out to be a secret legal genius.

That last one is, of course, the “Legally Blonde” heroine Elle Woods, a fashion major who decides on a whim to go to Harvard Law School and discovers her unconventional qualifications give her insight that her more buttoned up classmates lack. Rex Simpson, the protagonist of “Space Cadet,” bears more than a passing resemblance to Elle, and not just because the actress Emma Roberts could play, at a squint, Reese Witherspoon’s niece. (Her actual aunt, Julia Roberts, played another scrappy underdog in “Erin Brockovich.”)

Roberts’s most famous work might be in Ryan Murphy’s shows “American Horror Story” and “Scream Queens,” in which her knack for playing a certain kind of queen bee — gorgeous, cruel, one crisis away from combustion — makes her a magnetic presence. She’s great at a caricature, elevating those characters to satire without diluting their sugary poison. That flair for exaggeration would seem to make Rex Simpson the right role for her.

“Space Cadet,” a comedy written and directed by Liz W. Garcia, is cast closely along the lines of “Legally Blonde,” with some beats lifted so clearly from that movie I started to wonder if they weren’t meant as jabs. Rex is a neon-wearing bartender in Florida who wrestles alligators and loves to party on the beach, but there’s more than meets the eye: She was a bit of a science genius in high school, and dreamed of being an astronaut. When her mother died, she turned down a full ride to Georgia Tech. By the time she attends her 10-year high school reunion with her best friend, Nadine (Poppy Liu), she’s down in the dumps over her failure to, uh, launch.

A chance encounter with a former classmate who now runs a private spaceflight company sparks something in Rex. It’s time to chase her dreams. So she pops open the NASA website and decides to apply to be an astronaut. One problem, of course, is that she has absolutely no qualifications for the job. But is that a real barrier to Rex, the woman who invented patent-worthy tanning mirrors?

The movie continues in this direction, sending her to NASA in a crop top to become an Astronaut Candidate (or AsCan, a moniker that provides more than a few jokes). Here is where the “Legally Blonde” comparisons come in. There is, for instance, a scene in a classroom where Rex doesn’t know the answer to a stern professor’s question, then one later where she does, demonstrating her growth. There’s a whole sequence in which people look askance at Rex upon her arrival at NASA, thanks to her peppy, kooky outfit that signals unseriousness.

Her cohort seems oddly familiar, too. It includes a pretty and high-strung mean girl (Desi Lydic), who is determined to take Rex down because she’s convinced she’s not qualified to be there. Rex makes a friend (Kuhoo Verma) who needs a few confidence lessons, and she gives them. There’s an overserious overachiever who just came back from six years of isolated Arctic research (Josephine Huang), a gay M.I.T. graduate who proclaims himself a “smoothie artist” (Troy Iwata), an unsmiling former special forces operative (Yasha Jackson) and a patriotic former military captain (Andrew Call).

Most of all, there are the two brilliant people overseeing the AsCan training program, Pam Proctor (Gabrielle Union) and Logan O’Leary (Tom Hopper), whose British accent is explained away in the screenplay by handing him dual citizenship. Logan is the love interest, of course, though I had trouble staying interested in him. His main draw, as Rex eventually informs us, is his accent and his glasses.

Like Logan, most of the characters feel more like familiar types than actual people — not uncommon in a fast-paced, lighthearted comedy. But that means there’s nothing surprising enough in the movie to prompt laughter. The jokes feel tired. The actors are mostly doing their best, but the screenplay too often leaves them mimicking comedy rather than performing it.

All of that extends to Rex herself. If the freshness of a “Legally Blonde” movie comes from how aspects of the character’s weaknesses (many hours logged in a beauty salon, for instance) turn out to be strengths, the problem with Rex is we don’t know her weaknesses. She’s a bartender — everyone calls her a bartender to foreground how not-qualified she is — but aside from her competence in high-pressure situations, that work experience has had little effect on her. We are told Rex likes to party, but that’s limited to some wholesome nights out with friends. She’s a smart girl, a generous friend, a kind daughter, and if she’s not exactly qualified to be at NASA, she seems able to easily overcome all the challenges there.

In other words, Rex gets short shrift as a character, a shame for an actress of Roberts’s particular talents. When the moment calls for her to give a rousing speech of encouragement, it’s full of vague platitudes that any kindergarten teacher could spit out, rather than hilarious Rexisms. (Her only catchphrase seems to be “dude.”) By the end, we know wrestling alligators gave her the skills to dislodge some space debris, but it feels like an afterthought.

The film ends with truisms about making the big leap and never giving up, which feels mildly incongruent with the plot itself. That doesn’t mean “Space Cadet” is unwatchable, but it’s the sort of movie that makes you want to go back and revisit the better versions. Comedy depends upon sharply rendered quirks and absurdly specific bits that keep you chuckling. Without that, “Space Cadet” is less a comedy than a dream of one.

Space Cadet
Rated PG-13 for some off-color jokes and a few swear words. Running time: 1 hour 50 minutes. Watch on Prime Video.