It’s unlikely to damage the camera. The D7200’s Fn button is a normal push button designed to be pressed repeatedly, and the camera body itself is built to handle typical use and minor abuse. Continuous light pressure from an elastic retention strap generally should not harm the button.

The main thing to watch for is whether that pressure causes any unintended behavior, such as waking or powering the camera, which could drain the battery. If that doesn’t happen, there’s little reason for concern.

If you want extra peace of mind, try slightly repositioning the camera in the bag so the strap rests somewhere other than directly on the button.