Actually, based on the number of pages written, St. Paul did not write most of the New Testament. He did, however, write about one quarter of it, which is still a significant amount. (In my Bible, the Letters of St. Paul take up about 57 pages and the whole New Testament takes up about 233 pages, which means that St. Paul was responsible for about 25% of the New Testament by volume.) Even based just on the number of individual works, St. Paul was responsible for less than half (about 48%) of the New Testament since only 13 of the 27 works of the New Testament claim to be written by St. Paul. This is, however, still a significant number of the New Testament works.
St. Paul’s is often cited in official Church documents. For example, even though the writings of St. Paul make up only about one quarter (about 25%) of the New Testament, about one third (about 33%) of the New Testament citations in the official Catechism of the Catholic Church are to the writings of St. Paul. Therefore, one could easily argue the Catholic Church bases its beliefs on St. Paul more than any other New Testament writer.
St. Paul also holds a very high place in the liturgy of the Catholic Church. At Sunday Mass, during the Liturgy of the Word, there are three Bible readings: the First Reading (almost always a selection from the Old Testament); the Second Reading (a selection from a non-Gospel New Testament writing); and then the Gospel Reading (a selection from one of the four Gospels). Even though the Letters of St. Paul make up only about 45% of the non-Gospel New Testament writings, by volume, the Second Reading at Sunday Mass is usually from the writings of St. Paul (i.e., roughly 70% of the Second Readings are from St. Paul). In other words, if you go to Sunday Mass, you will probably hear something from the writings of St. Paul. Also, at any Mass, when Eucharistic Prayer 1 (the Roman Canon) is used and the saints are honored, Saints Peter and Paul are mentioned together before any of the other apostles. Likewise, whenever the Litany of Saints is prayed, Saints Peter and Paul are invoked together before any of the other apostles. Saints Peter and Paul are remembered together on the same feastday, June 29.
Based on these facts, I would say the Catholic Church considers St. Paul to be quite important indeed.