Does taking courses of various subjects matter in PhD admission?
If you are attending universities outside of the US, you normally wouldn't have much in the way of diversity—almost all of your courses are directed toward the general area of your major. So it's not that big a deal not to have too many outside courses.

In general, though, I also think that admissions committees do not place as much weight on courses outside the major as courses in the major. If you are a physics major, for instance, a C in a US History course is not going to be as problematic as a C in electricity and magnetism.