While personality-typing by astrological sign is not as serious as Ouija boards or actually consulting astrologers, to give credence to the idea that human beings can be sorted by the time of year they were born is, at best, superstitious. Here is what the Catechism of the Catholic Church (2111) has to say about superstition:
Superstition is the deviation of religious feeling and of the practices this feeling imposes. It can even affect the worship we offer the true God, e.g., when one attributes an importance in some way magical to certain practices otherwise lawful or necessary. To attribute the efficacy of prayers or of sacramental signs to their mere external performance, apart from the interior dispositions that they demand, is to fall into superstition.
At worst, even though astrologers or horoscopes are not actually consulted, giving credence to astrological signs can open one to the dangers of astrology (CCC 2116):
All forms of divination are to be rejected: recourse to Satan or demons, conjuring up the dead or other practices falsely supposed to “unveil” the future. Consulting horoscopes, astrology, palm reading, interpretation of omens and lots, the phenomena of clairvoyance, and recourse to mediums all conceal a desire for power over time, history, and, in the last analysis, other human beings, as well as a wish to conciliate hidden powers. They contradict the honor, respect, and loving fear that we owe to God alone.
Aside from the spiritual and moral problems attendant to superstition and divination, classifying people’s unique personalities by their birthdate unfairly stereotypes them. By asking their “sign,” instead of getting to know them there is a temptation to think: “Oh, you’re a Capricorn! I know all about Capricorns; they’re super-achievers!” This may have little or no correlation to the particular Capricorn you have just met who may be perfectly satisfied with his ordinary achievements and feels no internal pressure to super-achieve. By giving credence to a stereotypical assumption about the personalities of people born between December 22 and January 19, you have prevented yourself from really knowing this particular individual.
As a side note, our personality is part of our soul and God creates our souls at the moment of conception. If popular astrology were logically consistent, which it is not, it would classify people by their projected date of conception, not by their birthdate.