Oh it’s a lot more then that. It’s a doctrine of the CC.
The Church has never formally defined whether she died or not, and the integrity of the doctrine of the Assumption would not be impaired if she did not in fact die, but the almost universal consensus is that she did die. Pope Pius XII, in Munificentissimus Deus (1950), defined that Mary, "after the completion of her earthly life" (note the silence regarding her death), "was assumed body and soul into the glory of heaven." [http://www.catholic.com/library/Immaculate_Conception_and_Assum.asp]
Although the doctrine of the Assumption of Mary may somewhat controversially [http://www.newworldencyclopedia.org/entry/Assumption_of_Mary]
Pope Pius XII solemnly defined in Munificentissimus Deus on Nov. 1, 1950, that the "Immaculate Mother of God, the ever-virgin Mary, having completed the course of her earthly life, was assumed body and soul into heavenly glory."
The Roman Church solemnly warns anyone who “should dare willfully to deny or to call into doubt that which we have defined (i.e. the Assumption), let him know that he has fallen away completely from the divine and Catholic Faith (Munificentissimus Deus).
And if you don’t think it’s important to Catholics you may need to consider some of these.
"Membership in the Militia means complete dedication to the Kingdom of God and to the salvation of souls through Mary Immaculate." -Pope John Paul II
"We have an advocate in Mary that the father, Son, and Holy Spirit cannot say no to. It is impossible for them to say no to her! -Fr. Stephan Scheier
Pope John Paull II has 'dedicated himself and his Pontificate to Our Lady.' He bears the letter "M" for Mary in his coat of arms. And his personal motto, which is embroidered on the side of his robes in Latin is the following: "TOTUS TUUS SUM MARIA", which in English translates to: "MARY, I'M ALL YOURS."
"Oh Lady, you are the refuge of all the abandoned; behold me here and now abandoned by all. You are my only hope. You alone are my only hope; you alone can help me; have pity on me." [Taken from the Glories of Mary. Chapter 7, Page 132. By St. Alphonsus Liguori. He was declared "Venerable", 4 May, 1796; was beatified in 1816, and canonized in 1839. In 1871, he was declared a Doctor of the Church.]
Mary did die - her body was perfect however and went to heaven.
All of us will be raised on the last day with our bodies on the second coming if we deserve it.
Certain saints have bodies that are not corrupt. St. Bernadette is still perfect in a glass casket in France.
Henry Newman is not corrupt under the basilica in Philadelphia.
Also look at the etymology of the word doctrine.
It means true teaching.