As shown at http://shofars.us/, the tip of the horn has been carved to form the Hebrew letter ש, "shin" or "sheen".
Shin is often used in Jewish symbolism, such as on a mezuzah case, to represent the word Shaddai, a name for God. During the priestly blessings, the priest would hold his hands in a form representative of shin. It can also stand for "shema," the word for "listen," an especially significant act when it comes to shofar.
While the simpleness of an un-adorn shofar has symbolic meaning, decorating a horn can be an expression of hiddur mitzvah, the mitzvah of making a mitzvah-object beautiful.