When I first saw the twelve zodiac houses in Buddhist literature, I felt compelled to research it. I did a small writeup:https://sites.google.com/site/dharmadep ... n-buddhism
It isn't just Tibetans who value(d) astrology. You find it in East Asia extensively as well, even from an early period (early 3rd century, which is remarkable all things considered). Nowadays the astrology bits in sutras are not taught so much in East Asia as far as I know, but they're there.
My question is, what do you think astrology and karma have to do with one another?
From a purely speculative point of view, I imagine if astrology is valid, then your birth sign reflects your "completion karma". There is karma which projects a future existence (where in the three realms you'll be reborn, i.e., the "projecting karma") and then there is completion karma which determines what kind of qualities that life will have.
Asanga defines these two types as follows in the Abhidharmasammucaya:
“The results of favorable and unfavorable actions are produced in the good and bad destinies (sugati, durgati). This also, through the projecting action (ākṣepaka-karma) and the completing action (paripūraka-karma). What is projecting action? It is the action by means of which the result of fruition is produced. What is completing action? It is the action by means of which, after having been born, one experiences good and bad results.”
If you accept this theory of karma plus astrology, then it is not so much fatalism, but just a reflection of past deeds.
At the moment, however, I personally neither accept nor reject astrology.