Whilst I disagree with this sort of stunt in principle - on this occasion I'm prepared to make an exception. The monument itself isn't being touched and the fact it is raising awareness about prostate and testicular cancer makes it seem quite appropriate given the obvious attributes of the Cerne Giant (sentence partially edited).

"The stunt aims to raise awareness of ‘Movember’, the men’s health campaign which supports prostate and testicular cancer and mental health.
The moustache was created by grass seeds from British Seed Houses and carefully tended before being placed on the Cerne Giant, Britain’s largest chalk hill figure, for the day."