> The Mona Lisa is objectively better than anything I have ever painted.
No it isn't. If I saw one of your paintings and liked it better, there would be no way for you to prove me wrong. My opinion might be highly unpopular, but that wouldn't make it objectively incorrect.
And yet, it's not as clear cut as that. There is some hardwired stuff in our brains (see e.g. the "bouba / kiki" effect [0]) but most people will agree that what is considered pretty, good, etc is cultural, not to mention changing over time - like architecture trends as mentioned elsewhere in this thread, but also painting styles.
That said, a few posts back a commenter mentioned that the Mona Lisa is "objectively better" than anything they made; while I won't comment on aesthetics and appreciation, the assertion that the artist is more skilled in the arts than the poster is something definitely objectively true, simply because of education and experience. That's no guarantee that the outcome is better, but still.
To your point, though, I think as you look around at McMansions and Wal-Mart there's nothing architecturally redeeming about those structures besides efficiency in building and internal climate control. We know the architectural styles are bad and there are (again to your point about skill) scientific ways we can measure this (Ann Sussman's work).
If you look at historical architecture patterns (mostly pre-WWI) they mirror how art has changed over time.
It's not that Impressionism came about and Renaissance became shit, it's that both are good and important stylistic contributions.
It's not that Gothic architecture came about then all the Romanesque stuff sucked. Both were good and unique.
We run into the same problem as "all art is equal" when we tend to express the belief that because we live in the time period we're complaining about that if only we waited a few hundred years we would recognize that our current building patterns are actually really good. That's just not the case.
Some elements of beauty are objective. As in, they are hardwired into our brains.
Some cities are more beautiful than others. Prague is more beautiful than Detroit.