Here it goes in context: "The projections for the long run deficits and debt have already been revised downward. Go us!"
It is a play on a cheer. At a horse race, someone who has bet on a nag called "Speed Demon" might shout, "Go, demon." Or at a football game, a fan of the Whyoming Worms might try to encourage the team with yells or chants of "Go, Worms, go." (I don't know for sure whether the name of the football team at the University of Whyoming is actually called the Worms.)
In other words, "Go, X" is a locution frequently used as encouragement to X by a supporter of X. The usage that you are asking about is (or was the first few million times it was used) a mildly humorous play on that stock phrase from sporting events. "We," presumably the citizenry, are being treated as a sports team and fiscal policy as a game. Moreover, there is deliberate confusion about who is to be encouraged and who is giving encouragement. The speaker is pretending to encourage others to continue their efforts while simultaneously taking credit for those efforts.
The exhortation "Go [entity]" is a simple encouragement to that entity to do well and succeed. Grammatically it is in the form of an imperative statement.
Probably the most common example is "Go team!" where team can be the name of a team.
etc. The named team is being encouraged or exhorted to win a game, a championship, a trophy, whatever.
When used as above in "Go us!" the expression is an encouragement to a country or society (you don't name the contextual state or country), and is probably somewhat sarcastic, in that instead of encouraging a win it is celebrating (?) less of a loss.