Regarding how much the graduate program rankings are indicative of undergraduate programs, I would say – they are, to a good extent!
Graduate program rankings indicate how accomplished the faculty is in research and the quality of graduate students being attracted. A school with highly accomplished faculty and high quality graduate students is not going to run dumb-downed classes for undergrads. So, classes, especially at more advanced undergrad level, are going to be demanding. Moreover, because many managers tend to have graduate degrees, bachelor degree holders of school highly ranked for its graduate program in the field are going to be viewed very positively.
What a graduate program ranking does not indicate is: 1) selectively of undergrad body, 2) student resources, such as faculty-student ratio, advising support, etc, and 3) quality of classes and resources available outside the major.
A good heuristic for judging the quality of an undergrad program is to combine the USNWR ranking of the entire university and the graduate program ranking for the field.