I'd say as a person, he can express his thoughts. For him to personally condemn the events is good. For him to say that if the federal government were reasonable, Netanyahu should be arrested.

I think declaring that he would try to use his authority as mayor to pull the NYPD into it against the will of the federal government is a bit more tricky. During a debate when every single last other person pandered to Israel he made the solid point that a Mayor's job is to tend to his city, not foreign affairs. This sort of undermines that point. Admittedly, technically he still would only get involved if Netanyahu came to NYC, but by declaring this so publicly it has had the effect of provoking Netanyahu, who seemed to be inclined to test Mamdani's words by defiantly coming to NYC with Trump if Mamdani were elected.