Is it acceptable to request information from US math departments about the job placement of recent PhD graduates?
You could email either the departmental graduate advisor, chair, or POI but be prepared to be disappointed -- either because they don't respond or they don't have the data you want:


Until you've been offered a position, you are just one of a hundred plus applicants. Your likelihood of getting a response improves when they are courting you not when you're an unknown student emailing out of the blue
Running this type of stats takes time and energy as many departments don't have a mechanism for tracking former students. We often only do this on a departmental level during an external review (if even then) and so the data can be several years old and not very granular
individual labs or professors may track this but their n can be so small as to be meaningless (is a prof who places 2/5 really better than a prof who is placing 3/8).