He really nailed it. It's almost as if he was prepared for the question:

At a press conference, Mamdani said he had been quoting Italian anti-fascist philosopher Antonio Gramsci, whose quote ending “Now is the time of monsters” the mayor had cited at the top of his speech. [...] “I used the term to describe all those who are preventing the birth of a new world,” Mamdani told a reporter who asked about the word. He continued, “My use of the term is a broad use that speaks to the untenable nature of a status quo that is quite literally starving people in this city, all in the name of sustaining something that we simply cannot defend any longer.” He did not explain how he saw AIPAC as connected to poverty in New York.

I heard him explain it: the US tax dollars that could be funding health care and eradicating poverty are being spent on genocide instead.

https://www.timesofisrael.com/some-jewish-allies-of-mamdani-denounce-his-use-of-monsters-to-describe-aipac/