I think angular art is simply an adjective applied to a noun and isn't used as an actual defined term in the art world.
As such, it may not make sense to find when it entered the language. But angular dates from 1598 and art from c.1300. Here's a snippet of the 1920 Highways and byways in London by Emily Constance (Baird) Cook:
Therefore, at Millbank, they are but rarely gowned in angular "art serge," and are but seldom be-spectacled and be-catalogued. Neither are the Hypatia like girl-lecturers at all evident.
And a snippet of a 1929 report on the The Caribou Eskimos:
On the North Pacific coast combs are used which, with their ornamented handle, may recall later Eskimo forms, with the difference which the angular art of the Northwest Indians must necessarily, involve.
And finally the Princeton Alumni Weekly (Volume 40, 1939):
These unsophisticated ballads have an imagery and charm, as appealing as the quaint and angular art of Gothic times, and will be indispensable reading matter not only for the student of literature, but for all who love legends, those delicate fairytales of an untutored society.
Radu Miro suggests it means art you need to look at an angle in order to make sense, but I think it's any art which exhibits stark, pointy, angular edges, such as is seen in this Google image search mplungjan provided, and is to be found in modern art.