Critical response
Kevin Thomas of the Los Angeles Times gave the film a positive review, praising Lyon's performance and drawing comparisons to Blue Velvet, adding: "You don't ... have to take Hello Mary Lou at all seriously, and it probably would be a mistake to do so. Certainly, it's not on the deeply personal, highly idiosyncratic artistic level of the David Lynch film, but it is a splendid example of what imagination can do with formula genre material." According to screenwriter Ron Oliver, the favorable review in the Los Angeles Times inspired him to relocate to Los Angeles two years later: "I remember sitting back down at my desk and thinking, I’m going to move to America—because they understand the sensibility of quality trash." Critic Joe Bob Briggs championed the film, heralding it as "probably the finest pure-dee old-fashioned dead-teenager movie of the last two years."
Juan Carlos Coto of the South Florida Sun-Sentinel also reviewed the film favorably, writing that "despite its lack of original material, this film is well-scripted, directed, and acted–and surprisingly entertaining." The Atlanta Constitution's Eleanor Ringel similarly noted, "for all its rip-offs, Hello Mary Lou is never a total chore to sit through. As vengeance-minded females go, Ms. Schrage makes Glenn Close in Fatal Attraction come off like a Girl Scout leader," adding that it serves as a "black-comic commentary on the whole notion of prom queens." Joe Baltake of The Sacramento Bee noted that part of the film's "lurid appeal [is] its presentation of teen girls as evil narcissists," and "high school life as the ultimate sociopathic playpen." He went on to describe the film as a "hack-'em-up version" of Cinderella. Soren Andersen of The News Tribune praised the film's special effects and horror sequences, concluding: "The inventiveness of Mary Lou's effects and the anger coursing through it make it an effective schlock shocker. It proves that there are still some genuine shudders left in a seemingly threadbare formula."
Vincent Canby of The New York Times gave the film a mixed review and described its extended "grand guignol" finale, writing: "Bruce Pittman, the director, and Ron Oliver, who wrote the screenplay, have constructed the movie as if it were a gourmet banquet for toddlers. From the first course to the last, it's all ice cream." Bill Cosford from The Arizona Republic called it "a badly made film, as awkward as can be, and long stretches of it make no sense whatsoever. Nor does it manage, as the better slasher films do, to re-create a high-school milieu of even passing authenticity." Ted Mahar of The Oregonian expressly disliked the film, awarding it a one-star rating and describing it as "not so much a story as an occasion for a glut of not necessarily related special effects." TV Guide awarded the film one out of five stars, praising the special effects and Pittman's direction, but ultimately deemed the film "all too predictable."
Numerous critics noted parallels and homages to other horror films, such as Carrie (1976), The Exorcist (1973), and A Nightmare on Elm Street (1984). Betsy Sherman of The Boston Globe gave the film a mixed review, describing it as a "miserly slice-and-dicer: Carrie without the bucket of blood," though she conceded it is "somewhat livened by the presence of Michael Ironside." Stephen Hunter of The Baltimore Sun alternately described Ironside's talents as "waste[d]" and criticized the screenplay as "rudimentary." The Philadelphia Daily News's Ben Yagoda panned the film, writing that it "can be credited with nothing other than providing temporary employment for a group of untalented individuals," and drawing comparisons between Peggy Sue Got Married (1986) and The Exorcist (1973). Richard Harrington of The Washington Post drew similar comparisons, writing that the film "may be derivative, but for the most part it's clever enough to trade on its sources with humor and class. It's Peggy Sue Lives on Elm Street, with dollops of Carrie, The Exorcist and a half dozen other genre stalwarts."
Hello Mary Lou: Prom Night II went on to develop a cult following in the years after its original release. On the review aggregator website Rotten Tomatoes, 56% of 16 critics' reviews are positive, with an average rating of 5.3/10.
In a retrospective assessment, film scholar and critic John Kenneth Muir wrote, "In the annals of unnecessary sequels, Hello Mary Lou: Prom Night II rates high. Contrarily, in the ranks of 1980s horror movies, it's merely a mediocre effort." Film scholar Mike Mayo said the film is only a Prom Night sequel by title, and that it in fact bears more similarity to A Nightmare on Elm Street (1984). Writing for Syfy in 2017, Rebecca Pahle praised the film's special effects and acting, and deemed it a "slasher masterwork."
Jacob Knight of ComingSoon.net similarly praised the film in a 2015 retrospective, writing: "It's a shame Hello Mary Lou never became a bigger hit, because it's an oft-forgotten gem of the horror genre, standing the test of time nearly thirty years on. An amalgamation of national tax shelter weirdness, brazen borrowing from better films, and the tossing of creative caution to the wind, Pittman's picture evokes numerous classics while indubitably carving its own identity."
