The mirror is flipped up before the shutter opens, the very basics of SLR. Whatever dust speck might sit there won't affect the picture directly.
A word of caution since there were already some people close to a heart attack: Don't wipe your mirror at all! The reflecting coating is at the front (in contrast to your bathroom mirror) and is quite delicate. Good chances to scratch it. If you suspect some dirt and it doesn't go away from a few blows better have it checked at the Service Center.