What kind of camera?
if it was a DSLR, then no!(unless they had mirror lockup for sensor cleaning enabled.
The mirror in a DSLR doesn't allow much past it to touch the shutter, so in reality if they stuffed a cleaning cloth in there, then the worst damage they could do is to damage the mirror in some way.
Of course it's easy to lift the mirror of an SLR against it's will by hand, so they could also have done that too.
I use canned air to clean out the mirror box area of my cameras, but hold the camera up high and facing down so that any loosened particles fall down(as I'm pretty sure that they still can't fall up!
)
Stuffing a lens cleaning cloth is not something I'd ever do, as I reckon it would serve no real purpose .. unless there was something obviously on the mirror, and it was the ONLY way to get it off.
I always have access to some thing or device(brush/ blower bulb) to do that kind of thing, so the use of a cleaning cloth in there is pretty much not going to happen for me.