It seems odd that airliners do not have an easy way to monitor critical components on the exterior of the aircraft, should for example, a bird or something else strikes hard enough to cause damage. A mirror or camera would allow a pilot to see and react to a possible dangerous situation quickly. Also, a camera helps a pilot to see what's behind.
Aircraft rarely collide with any objects. Cameras would only be helpful if the extent of the damage is visible from the outside. Instruments, on the other hand, can be used to diagnostic the problem even if the system fails "internally".
E.g. Many hydraulic lines and cables run across the wing. If these is damage on the leading edge, is it the slats? A hydraulic leak on system A? system B? Is the de-ice working? You need instruments to get a better picture.
Some aircraft indeed come with cameras. The Boeing 777 has cameras on the bottom of the fuselage and the tail to aid pilots during taxi operations (the 777, especially the 777-300 variant, is really long...)
The Airbus A380 also has a tail mounted camera. Passengers can enjoy a spectacular view of the clouds, the high speed takeoffs and landings:
As for mirrors, pilots don't overtake other planes or back off like drivers do. There really is no need to know what is behind the aircraft. Pushback is accomplished by the tug on the ground, who has a much better view of the area than the pilots.