Mirror view by default
In Issue #1 (moved), the question whether the view could be mirrored was answered by referencing the "flip" effect. This technically works fine, however, I'd propose making a flipped view the default when using a webcam or a front-sided camera.
The reason is that seeing yourself in an un-mirrored view is very uncommon: Most smartphones (including Android and SailfishOS), most video conference software and, ultimately, real mirrors mirror the view while you look at yourself. Seeing yourself in an unmirrored view can be far more confusing than the other way around.
Treating the mirrored view not as an effect, but as a separate option, ultimately makes the difference that it could be applied to all other effects as well. AFAIK, choosing 2 effects (flip + any other effect) is not possible.
If it is not easy to automatically determine whether the camera is on the back-side of the device (like on Librem 5 or Pinephone) or whether it is on the front, I'd argue for an easily discoverable switch and the flipped option as a default – because using Cheese on a phone will currently rather be the exception, while using it with a webcam will be the most common use case.