Ok, there's my answer! Cherry-Picker originates in AH itself.
Nope, your first theory was correct. The term was in AW in the 90's.
There is nothing "wrong" with picking per se. It is a perfectly valid and realistic tactic. It's the social aspect of it that causes the rub. It doesn't really take much skill to shoot down someone who is already occupied with other bogie(s). There's a little bit involved (predicting the opportunity, timing and approach), but not much
If everyone involved is aware that it was just an easy kill by an otherwise skilled pilot, there's nothing to disagree about. The stigma kicks in when the shooter has a self inflated sense of skill (eg. thinks that kill is equal to killing that same pilot 1v1), or when the depth of the shooters ability is limited to finding easy kills due to their lack of skills.