The roaches are snug and warm behind the mirror. Some gel and bait will take care of the roaches. The gel comes in a syringe that makes it easy to use. If the gel can be inserted behind the edge of the mirror or however it will kill the roaches. The roaches will eat the gel. Combat will kill all kinds of roaches.

Call an exterminator.

I have a large bath room mirror that has lighting behind it leaving about a 3in gap between it and the wall (the mirror takes up a whole wall) and there are cockroaches behind it falling from it and stuff, please how can I get rid of them