It is a brilliant idea; however, technically it's still vandalism. Vandals deface something that is not their own. An offended owner still must spend money and energy to rectify the offense, whether that be removing paint molecules on the disturbed area or removing dirt molecules on the undisturbed area.

By default, the owner of a dirty wall is saying, "I like it the way it is". A "soap artist" uses the false premise that cleanliness is a virtue in order to steal real estate to promote his message.