Trump specifically said that Mamdani should be arrested if Mamdani "interferes" with ICE enforcement operations.

He did not specifically say that. He was asked to respond to Mamdani's comments in his acceptance speech as follows:

Mamdani vowed in his recent acceptance speech to use his power to "stop masked ICE agents from deporting our neighbors."

Asked during a press briefing Tuesday what his response is to Mamdani's promise, Trump said, "Well then, we'll have to arrest him."

axios.com - Trump mamdani arrest ice immigration

He didn't say, "we'll have to arrest him if he interferes" with ICE. Or that we'll have to arrest him if he breaks the law. He made a general comment that if Mamdani does what he says, that they'd have to arrest him.

It strains credulity to believe that Mamdani is talking about taking personal action to "interfere" with ICE agents. A more plausible reading of his speech is that he will use the powers of the office of Mayor to try to "stop masked ICE agents from deporting our neighbors." Since there presumably exist some lawful powers that inhere to state government to do that, and since it's not especially likely that Mamdani is deliberately stating - in public - that he intends to commit crimes, the plausible reading of his campaign speech is that he intends to use whatever lawful means exist for pushing back on ICE tactics.

You know, the same way that Trump has combed over all of the powers of the Executive to find all the ways he can use already-delegated federal powers to try to accomplish his goals, even in pushing back against what private actors and/or state actors are doing.

Which is why there's such negative reaction to the statement. Had a Democratic President made a similar comment about a conservative state official promising to use their office to try to resist something conservatives disagreed with, it would have elicited a similar negative reaction among conservative observers.