Art is defined as (whatever).
Sorry, no. Art is all about that very human trait to scratch things on walls, fling faeces in pretty patterns, whatever. Some of it is going to be "good" art, some of it is going to be "bad", who knows, and who gives a flying? Some artists will be lucky enough to have their work appreciated and paid for in their lifetimes, some will have to wait until after they are dead, some won't and some won't ever be "appreciated" (ie, their art is good only in their own head, which is where it all starts).
In this case, I reckon it would have been pretty awesome to see, I'd hope it didn't do too much damage as it was taken off, but if it was equivalent to, say, 10 years of wind and rain, where's the harm in that? Chateaux and castles are 10 a penny, making it an alternative experience is no bad thing.