most built structures have never been commissioned by an architect--they're too expensive!
almost everything in the urban landscape is engineered to support X weight for the absolute least cost that will pass the local building codes.
there is nothing inherently beautiful about the urban landscape in terms of structure.
there is nothing inherently beautiful about graffiti, but sometimes a great notion.
given the above, I posit that it is more likely for graffiti to be beautiful, as it is simpler and orders-of-magnitude more cost-effective to create a "winner." Hell, that's including bail money.
to my mind, my theory is more than borne out by experience. but I admit a heavy bias.