As usual, this has been a top down process. It began about a century ago after World War I with the advent of Modernism is art - in fine art with the likes of Picasso, in Literature with TS Eliot and James Joyce, in classical music with Schoenberg and Stravinsky, in architecture with Le Courbusier and Bauhaus...
These massively influential artists were unlike the geniuses of previous generations because they were not even trying to create beauty.
After World War II this became mainstream; ordinary local government officials, administrators and managers of all kinds became united in their indifference to beauty, even disdain for 'conventional' ideas of beauty, which was regarded as kitsch, outmoded, probably patriarchal and racist too.
The indifference to beauty was initially excused by claiming a focus on efficiency, cheapness, egalitarian social engineering; or that the pursuit was of a higher and more contemporary and forward-looking form of beauty... but it very soon became clear that the motivation was in fact a hostility to beauty.
This can best be seen from Architecture, that most public of arts.
What is significant is not so much the ugliness of modern buildings and the built environment - ugliness has always been common, especially where there is poverty; but the fact (and it is a solid, established fact) that people are not even striving for the beautiful.
In terms of the externals; universities like Oxford and Cambridge were international magnets for students, faculty and visitors because of the beauty - yet they consistently themselves built (and allowed) the erection of vile and brutal buildings among and nearby the loveliness. e.g. the Cambridge History building from 1963:
At my alma mata Durham - similarly an architectural jewel; in 1966 the expanding university built a brutalist, grey, grime-streaked concrete box as their students union.
This has, of course, continued and increased - everywhere. In situations where there was every reason - every commercial, monetary, expedient, materialist reason to aim for beauty; ugliness was insisted-upon.
The same applies to the insides of buildings. Instead of trying to make public work and life environments pleasing, cheerful, beautiful - the choice was to aim for bleakness, oppression, noise and lack-of-privacy.
In recent decades, even when building offices for themselves - the places where they will spend many hours per day; management have created nightmarish 1984 conditions. Below is the aptly named Portcullis House, the business annexe to the Houses of Parliament, where Member of Parliament have their meetings etc. They actually made this for themselves,
This cost a quarter of a billion pounds 20 years ago: the physical manifestation of an aspiring global totalitarian regime
What can we conclude? As William Arkle said: "If the individual is degenerating then his idea of beauty will be degenerate also".
Beauty is an index of the soul; beauty(with truth and virtue) is one of the prime transcendental values, and an indifference to beauty is a sign of the canker of corruption already present and growing in a person or a group.
By the time active and sacrificial hostility to beauty has become manifest, we are deep into the inversion of values; a situation when beauty and ugliness have changed positions, and evil is being pursued purposively.
We see it all around us. This is a world where Satan rules.