I wouldnt be able to get into a position to do that. So my carers check daily. I did used to use my mirror on an old microphone stand but still needed carers to turn me.
I think a phone screen is a bit small to see much very useful. And they keep going off/dark while you try to look to save the microscopic battery! (my partly serious dig there as I cant stand the things! Yes on PC.) Would you be able to see the shades of red/pink etc well enough? Maybe with a decent background light.
Would it work for you? Well anything is better than nothing. Sometimes though its hard to tell much on a picture especially if you were just laid on it. You really need to press it and see if it goes white, or feel for hard areas etc as well to decide what it is that you are actually seeing. What you need is a volunteer!