It's not a good thing they have a nuke, considering every country in the middle east hates them and Iran hates them in turn (forced to throw blindly at a dartboard of Middle East countries, Iran would probably be happy regardless of who it hits) and it would certainly make them more of a domineering figure in the region, but America is the least pressing target.
For precedent: Pakistan have nukes (40+), are a close friend of Iran, had Osama hiding within their country and never informed the US, and have deep ties to China. Their neighbour, similarly, possesses nukes and hate each other. Despite this, they've never been used, never been covertly delivered to terrorist groups.
Iran is a threat according to the Trump admin but only because their increased influence would probably force their neighbours into conceding to their influence, which is less likely for some (Saudis, Egyptians, Iranians, Jordanians) than others. Your care about the region depends on whether or not you give a shit about the Middle East and if you worry this'll have knock-on effects for things like petroleum production and regional unrest.