At Metacritic, which assigns a weighted average score out of 100 to reviews from mainstream critics, Mumps, Etc. received an average score of 62% based on 19 reviews, indicating "generally favorable reviews".
Mike Lechevallier of Slant Magazine said, "Mumps, Etc. isn't a career-defining moment like Alopecia, but it's a celebratory return to form for Why?, reuniting the band with the key attributes that ignited their creativity in the first place." Ryan Reed of Paste called it "their most layered, headphone-friendly set to date, utilizing an eight-member choir and a string quartet, not to mention plenty of harps, flutes and ass-blasting rhythms."