Perhaps we're seeing the American way of casual dressing influencing Argentine style. I appreciate how older Argentine women take the time to dress, wear makeup, etc. for a trip to the grocery store. There is still a generation here that hasn't been influenced by American casualness.
The under-40 generation goes casual because they can't afford anything else. I see another side of formality in the milongas where the over-60 generation dresses to show respect in a social situation and for the dance, the way it's been for decades in Buenos Aires.
I'm not sure it's directly a US casualness or a global casualness that was sparked initially in the United States. There are lots of young people that can afford to dress very stylishly and be immaculately "made up" for everyday activities - and Argentines are no different. It's not so much a question of cost in many cases as cultural "value" (not values). Younger people just don't value that, and I'd say that's as true almost anywhere.
There's also a difference between functionally/stylishly casual and too casual. I'd say that the average under-40 Argentines are better than their U.S. cousins on the aggregate in this respect. It's understandable that the poor in Argentina are limited in their options, and its not fair to paint them with the same brush as people in the U.S., most of whom don't have this excuse. You can buy all your clothes at any number of low-cost retailers in the U.S. and still look presentable, even in a casual context. As internationalguy said, it's the socially acceptable irreverance for any effort at style that seems to pervade much of US society.