depends on what you mean by postmodern
yes, they are architecturally after modernism
no, the buildings don't look like the ATT building or PPG place in pittsburgh.
eg no columns, turrets, castles, chateau roofs, etc
very little of this in Atlanta:
or this
or this
atlanta's 1980s and 1990s scrapers are more neo-futurist (thanks to portman)