How to tell if my iPhone is hacked?

The popup is a phishing scam to steal your money, your identity, or both. Ignore and dismiss it, and don’t tap anything in the popup, including “close”, done, or any other buttons on the popup. Here is more information→Avoid phishing emails, fake ‘virus‘ alerts, phony support calls, and other scams

Your iPhone has not been hacked. There are no known ways to hack an iPhone remotely. The only way it can be hacked is if someone who knows how to has possession of the phone while it is unlocked for at least 30 minutes. If this has actually happened restore the phone to factory settings and do not restore a backup; set it up as if it is new→Restore your iOS device to factory settings - Apple Support