Jokes are jokes. If cabin crew say that smoking is permitted outside of the aircraft during flight then its obviously a joke and I can understand and appreiciate that. To fool paying passengers into beleiving something that isnt true is unprofessional and of such an extent he doesnt deserve to be in charge of an aircraft with passengers on it ever again.
I would go further than that. If I say to airport staff or cabin crew that I have a bomb or was hijacking the aircraft even if I told them that I was joking I would expect to be locked up and criminal charges made. There are notices up in airports that say that any comment will be taken seriously even if made in jest. Dismissal is a light alternative to a criminal charge.