Currently Mycelium on Android lets you create watch only addresses by scanning the public key QR code and import paper wallet private keys. But the iPhone iOS version doesn't. What apps are there for iOS that lets you do watch only and import from a paper wallet?
Breadwallet can sweep private keys: https://www.youtube.com/watch?v=WJ7-4Hv5Kq4
AFAIK, there are no iOS wallets that can import private keys.
In case you are looking for an iOS app to only watch a bitcoin address (ie. you don't have the private key, or it's on paper or in a hardware wallet), you might be interested in ocular.
Fully Noded iOS is a fantastic option for this I believe. Very powerful mobile iOS wallet for controlling your bitcoin/LN node.