Will the pixels be visible ? Will the thickness of the screen glass prevent the image from being in focus ?

Some resin-based 3D printers work on a similar principle: an LCD monitor mounted under the transparent bottom of a tank holding light-sensitive resin exposes the resin and causes it to solidify. There's no doubt that you could make a contact print by placing your phone or tablet on a sheet of photo paper to expose it; what's unknown is what kind of quality you can expect.

You've already identified one problem, which is the separation between the surface of the display and the paper that is caused by the device's glass cover. Devices vary in the thickness of that cover glass, though, so some may work better than others.

Another variable is the exposure time, and whether you can arrange to make the image appear only after you've placed the device on the paper. If you just display the image and then place the device on the paper, you'll probably get a bit of blurring because the device moves at the beginning and end of the exposure. If you have the technical skills, you could write a program that makes the display show all black, and then shows the image during the exposure, and then goes back to black afterward so you can remove the device.

Important: Remember that you're working with a negative process here... if you're making a black and white print, for example, the regions that are exposed to light will turn black. That means that you need a negative image in order to produce a positive print. You might be able to get something workable by using a feature on your phone that inverts the display (iOS supports this under the Accessibility options), but keep in mind that that feature is meant as an accommodation for a disability and it might or might not work well for printing photos. An app built for this purpose would be a better plan.