Is it possible to wirelessly debug iOS app on iPhone?
It is possible to debug iOS apps on an iPhone wirelessly over WiFi, yes. This is a supported way of debugging in Xcode.

With Xamarin you can also debug over WiFi if you enable "Debug over WiFi" in the "iOS Debug" Build settings. However with Xamarin you need to launch the application over the USB cable. So you cannot do a fully wireless setup there.

Doing wireless debugging with a "cloud Mac" would probably require setting up a VPN connection or similar that makes it appear as though the cloud Mac is on your LAN. And it would only work with Xcode - not Xamarin.

A different approach would be to buy for example the Eltima USB Network Gate. Connect the iPhone to your local PC and use the software to connect that USB port over the network to your "cloud Mac" where it would appear as though the iPhone was physically connected. This should (at least in theory) allow you to launch and debug apps using Xamarin.