Brutalism is absolutely an acquired taste, but it is, by all measures, an objective style that can be measured against, in the same way classical or say, Spanish Mission architecture style is. In fact, some would even go so far as to argue that most Victorian/neo-classical architecture is just a chintzy facade with no real architectural rules, akin to a Vegas casino or a Cheesecake Factory (I wouldn’t, but some would).
But as far as Boston City Hall being “objectively” ugly— that’s an objectively wrong statement. I can understand the vast majority might unfortunately say they personally find it ugly, but that is subjective.
I have the same views on e.g. Dallas City Hall too, which I think is another stunning example of a beautiful building many people hate.
Saying a pile of trash covered in mold and feces is objectively ugly— I’d agree there. Barring that, though, I’m of the strong opinion that most anything expressive, including graffiti, is “art”. Your response to whether you find it beautiful or not is your own, but that does not make it art or not.