From what I can tell Mamdani has been speaking our against these events. So he's spoken about Gaza at least once since election... Though I am neither from NYC nor the US, so don't know in great detail.
Again, Mamdani isn't a socialist, and isn't gonna bring about any sort of meaningful restructure of US society. But it seems like a stretch to say he's actively supporting US imperialism given the scope of his power.
Lesser evilism only really applies when no other choice exists. In the US voting system, you must vote for the lesser evil, because first past the post is trash. The part that people haven't been doing at large scale is still continuing to bring about change outside the terrible voting system, 1. at a minimum to fix the voting system and 2. to build workers movements that use things like, unionism, general strikes, protests, boycotts and eventually revolution to overthrow the ruling class by (mostly) peaceful means by numbers (and probably some violence when there's inevitably a retaliation by the ruling class).
But not voting in the US and feeling high and mighty about it is really stupid.
Luckily, in Australia, we can at least preference, meaning we don't need to deal with that particular hurdle. Though it's still not democratic as representatives are unrecallable, and like all capitalist counties, the real power is with the capitalist class who put their finger on the scale to get their desired outcomes.
Lesser evilism in the context of world governments is absurd (i.e. supporting Russia, China, Iran, etc just because they're not the USA), especially socialists who believe in international worker solidarity.
We get absolutely nothing but supporting totalitarian regimes, be they in the US, Russia, China etc. We ought to be supporting workers in all countries to achieve socialism, not bloody the ones in power just because their ruling class are enemies of the US ruling class... (and for the record, China, Russia and Vietnam are all capitalist countries... one needs to delude themselves to believe otherwise).
In conclusion, it's weird that you don't mind being called a tankie, because it hints are your disregard for democracy and self-determination.
And by definition you can't really call yourself a socialist if you don't believe in democracy, since how else can a society be "worker controlled" if the workers don't get a say over society....