I think the point is that gentrified areas are prim and proper and tagging pushes back on the gentrified aesthetic.

There’s also the phenomenon of guerrilla gardening beautifying a neighbourhood, which can cause developers to take note of it as a “desireable area” and bring luxury development. Tagging has the opposite effect.

I don’t know enough about Berlin to comment there, but it can be true that city can still have a high average rent, at the same time as tagging a building and the surrounding area can decrease rent by deterring rich people.