I'm trying to find the exact page where I located this answer, but unfortunately it's not explicitly stated in any of the places one would expect (damage or the zombie chapter). Still, it's hinted at in the Flesh Notes on p.147, and I know it's explicitly stated somewhere else in the book (in passing). Anyway, the short answer is: no. Zombies do not suffer multipliers based on hit location. Note that, this being the case, handguns of a caliber of .38 or less will normally need at least two shots to put a rotter down.