Yes, you can connect a USB mic to your iPhone. There are two ways to do this: either through the Lightning port or via the headphone jack.
If you have an iPhone that has a Lightning port, you can use Apple's Lightning to USB Camera Adapter to connect your USB mic. Simply plug the adapter into your iPhone, and then plug your USB mic into the adapter. Your iPhone will then recognize the USB mic as an audio input device.
If your iPhone does not have a Lightning port, you can still connect a USB mic by using an adapter that plugs into the headphone jack. These adapters typically have a female 3.5mm jack on one end and a male USB Type-A connector on the other end. Once you've plugged the adapter into your iPhone, simply plug your USB mic into the adapter and it should work just like if it were plugged directly into your iPhone's Lightning port.