Not a Ryanair exclusive trait I'm afraid.
A few years ago when Flybe started CWL-EDI in competiton with BMi Baby, they hired the Bay City Rollers to sing their hit "Bye Bye Baby" to the first set of boarding pax. Admitted this was more like a little inter-company ribbing rather than the nasty undertones of daubing one's aircraft like that.
There is a thread on one of the forums about how ryanair are now running a competition for pax to come up with new ways of charging their punters! Lowest common denominator.