"I was ashamed when I heard the name," the pontiff told an audience of students at the Vatican.
"A mother gives life and this one gives death, and we call this device a mother. What is going on?" he asked.
Last month the US dropped such a bomb, which weighs 21,600lb (9,800kg), on Islamic State militants in Afghanistan.
The Pentagon said it was dropped from a US aircraft in Nangarhar province, targeting tunnel complex used by IS.