Is it appropriate to ask professors to give you a mock interview? Suppose that an undergraduate student is preparing for interviews for Ph.D. positions in natural sciences. Is it common/acceptable to ask a professor with relevant expertise and familiarity with the student's educational background and research ability to conduct a mock interview with the student as a way to ascertain problems in the student's preparation? Or will such requests be viewed as unnecessary or "asking for too much"?

I'm primarily interested in the acceptability of such requests in US colleges.
It depends on the professor. If it's someone who knows you well and has the time, then they would probably be happy to help. Most of the professors I came across during my undergrad were more than happy to help preparing me for grad school interviews. Just make sure you ask well in advance, so they have time find an appropriate time, and don't take it personally if they say no: professors are really busy. 

EDIT: Make sure you ask professors that have been through the graduate application process in the same country you are applying, if possible. Ones who received their PhD elsewhere, or say 50 years ago, will probably turn you down simply because they don't feel familiar enough with the process (unless they are directly involved in admissions for your school's graduate program).