Yea i agree Middle Seat injecting humor into the demo itself i find needless, and unproffesional. It detracts from the importance and gives the wrong opinion i feel.
In the UK anyway and with my airline, the PA's that are issued are either inside a black box indicating that nothing in this box can be changed as it as been reviewed by the CAA and this is what it is we must say, nothing added nothing taken away. Any PA not ringed with a black box means ad libbing were needed is fine, ie you can add humor here.
I was under the impression that if a CAA inspector was onboard and you were adding bits or taking things out of the demo you could loose your license to fly.
Its bad enough trying to get some respect out of the pax never mind trying to do a stand up routine during the demo!