Punitive criminal systems are notoriously bad at reducing crime, actually.

Honestly, I had expected that it was a minor improvement (and it was - something like three fewer murders), and insofar as I would have seriously credited Mamdani at all, I would've attributed it to the general positive mood after Mamdani's victory - he certainly is very charismatic and active in the community, and such things do have an effect in the public mood and awareness.

However, someone else pointed out this was, apparently, the coldest January in the past decade, so that's the more likely culprit.