Upon release, "Slap-Happy" received generally mixed reviews from music critics. Marc Weingarten of "Rolling Stone" praised the band for "doing punk metal right", commenting that "Slap-Happy" "is all hopped-up, pared-down riffage with no apologies or gratuitous change-ups." "Q" magazine highlighted the album for its "distinctive punk noise", which "stays roughly the same but evolves enough to be interesting." In contrast, Craig Daniels of "Exclaim!" criticized the album's sound for being "sterile and lacking in energy" compared to previous L7 albums, but overall considered "Slap-Happy" to be "a fairly solid record".