There is precisely nothing about "being an art major" that gives you any more or less right to an opinion. Especially if you actually had read anything about current philosophy of art you would realize how dumb that sounds in this context. Since "postmodern" theories of art focus on the inner expression of the artist, contrasted with the subjective experience of the art by the viewer.
I am the one who has to suffer through this every day in the train. How am I not entitled to an opinion about that "art"?
Historical buildings is a different matter of course.
The actual art is often a bit strange but can be beautiful, and is more interesting than a bare concrete wall or train hull. But I agree with you that virtually all tags are ugly. And not only that, tags represent an extreme concept of ego, where you subsume the art, spending your entire canvas on your signature.