This has been a long pending feature, and I finally have a fix. Yay!
Please see details at:
View the iPhone Camera in Landscape on your Mac
Essentially screen mirror thinks the camera app is in portrait mode…because it is. On android and iOS devices the camera is a portrait application that does not change orientation when you rotate the phone, it just creatively rotates the text labels.
This is just like the app launcher which doesn’t care how you hold the phone, it shows the icons in a portrait mode.
There isn’t a clean way to detect that the Camera app is running and rotate the screen. Let me know if this fix works for you. And if you have other feature requests.