We are dealing with two pontiffs here, Pius VI and Pius VII.

Pius VI was pontiff from 1775 to 1799. Formerly Gianangelo Braschi, this pontiff was mainly concerned with issues on the continent. His encyclical Inscrutabile (English translation) was highly critical of Enlightenment ideals. How critical? Well, here's an excerpt:

When they have spread this darkness abroad and torn religion out of men's hearts, these accursed philosophers proceed to destroy the bonds of union among men, both those which unite them to their rulers, and those which urge them to their duty. They keep proclaiming that man is born free and subject to no one, that society accordingly is a crowd of foolish men who stupidly yield to priests who deceive them and to kings who oppress them, so that the harmony of priest and ruler is only a monstrous conspiracy against the innate liberty of man.

Everyone must understand that such ravings and others like them, concealed in many deceitful guises, cause greater ruin to public calm the longer their impious originators are unrestrained. They cause a serious loss of souls redeemed by Christ's blood wherever their teaching spreads, like a cancer; it forces its way into public academies, into the houses of the great, into the palaces of kings, and even enters the sanctuary, shocking as it is to say so.

So how critical? Very critical.

While the US was an ocean away and received little attention aside from procedural issues such as establishing diocese and the like, the French Revolution beginning in 1789 was seen as a catastrophe. The papacy and the French government antagonized each other for a number of years, eventually leading to armed conflict and a defeat for the papacy. Eventually, the conflict flared again and the Roman Republic was established, but Pius VI refused to back down. He was declared deposed by the French, and taken to southeastern France. Upon his death, he was denied a christian burial. His death was recorded in the town hall of Valence--occupation: pontiff.

So, the first of our two pontiffs who dealt with the earliest years of America was clearly not a fan of the Enlightenment or revolutions. His successor, Pius VII (pontiff from 1800 to 1823), was quite a different figure. While a cardinal, then Barnaba/Gregorio (he went by both names) Chiaramonti published the following words in a homily:

The form of democratic government adopted among you [the french occupiers of his region], dearly beloved, in no way contradicts the maxims I have previously stated, nor is it repugnant to the Gospel; it demands all the sublime virtues that are learned only at the school of Jesus Christ and that, practiced religiously among you, will make for your happiness and contribute to the glory and renown of our Republic...Yes! My dear sisters and brothers, be good Christians, and you will be excellent democrats.

(A History of the Popes: From Peter to the Present. John W. O'Malley, S.J. pg. 233, translation his)

In this same sermon, he quoted Scripture, various Fathers of the Church, the Roman historian Sullust, and Rousseau. This man clearly did not share his predecessor's views on the Enlightenment. That said, he was no revolutionary. His major achievement was early in his reign: a Concordat with Napoleon. Neither party was entirely satisfied with the arrangement, but it dominated the dealings between France and the Church for decades to come. Later, he would reestablish the Jesuits (recently suppressed) and lived long enough for the Congress of Vienna to restore the Papal States. He maintained official neutrality while Spain battled what would become South American republics, but he came nowhere near supporting their cause.

He did have a somewhat famous quote about the US, but it was in reference to the US efforts in fighting the Barbary pirates. He said the US "had done more for the cause of Christianity than the most powerful nations of Christendom have done for ages.” This is a fairly utilitarian statement, and shouldn't really be taken as effusive praise.

Simply put, the United States was very much a sideshow in the Vatican's eyes for much of its early history. Major changes were taking place much closer to home, and the papacy concentrated on these crises rather than yet another upstart nation an ocean away. There may be more interactions between the papacy and the US that I am unaware of, but the Americas were largely seen as much less important than the massive changes in Europe.