MILWAUKEE—Defenders of the Milwaukee city flag love its picturesque collage of factories, Lake Michigan, a church, giant barley stalk, clock tower, steamship, sports stadium, Indian chief and genie’s lamp.

Haters point to the same things.

“It’s a hot mess,” said flag-obsessed designer Roman Mars, in a TED talk critique that drew more than three million views on YouTube.

The North American Vexillological Association, which tracks flags, ranked Milwaukee’s 147th out of 150 cities surveyed. Only three were worse: Rapid City, S.D., Huntington, W.Va., and Pocatello, Idaho.

“Nothing can quite prepare you for one of the biggest train wrecks in vexillological history,” Mr. Mars said of Milwaukee’s flag.