When any object is attached to the outside of an aircraft it results in an aircraft that no longer conforms to its type design.

14 CFR 3.5(a): "Airworthy means the aircraft conforms to its type design and is in a condition for safe operation".

My guess is the aircraft would not be considered "airworthy" by the FAA (USA, of course). I suppose this leads to a "don't ask, don't tell" situation.

Is it safe to attach a camera to the outside of an aircraft? I don't know.

Is it a good idea? I don't think so.