"Ugly Heart" received general acclaim from contemporary music critics. A writer from Popjustice branded it a "really quite brilliant song about hot wankers" and gave it a 9 out of 10 rating. The writer believed the song was better than the group's debut and improved their chances of a successful career.[13] Jamieson Cox from Time described the song as "moonlit, guitar-oriented pop" and a "sneering kiss-off" with many "undeniable" features of a Dr. Luke production. He also praised the band members chemistry on the song.[14] Mike Wass from Idolator thought the track was an "irresistible anthem" that left him "hooked" after one listen. He particularly liked the "guitar-pop direction" because it differed from the more dominant charting electro-pop songs.[15] Perez Hilton described the song as infectious, great and awesome. He likened the beginning of "Ugly Heart" to a signature Jason Mraz track and praised the lead vocals.[16]
Homorazzi.com reporter Donovan Pagtakhan named it the group's best song to date and a "catchy pop track with a folk vibe",[17] while Maximum Pop! said the track "feels like a tune you'd pump in your car when riding with your girlfriends.[18] Brad Stern from MTV labeled it a "certifiable, Dr. Luke-produced smash" packed with "chant-along-friendly hooks".[19] Jason Lipshutz from Billboard quipped "the group uncovers the winning pop formula that previous single 'Vacation' failed to unlock."[6] Sugarscape.com's Carl Smith dubbed it an "insanely sassy single".[5] Igee Okafor writing for magazine The Source likened "Ugly Heart" to "up-tempo and high-energy" releases by Ke$ha. They described the ukulele as "infectious" and the vocals as "powerful and impeccable".[21] Billboard named the song #50 on their list of 100 Greatest Girl Group Songs of All Time.[22]