My feelings on grad school rankings are the following:
- If a grad program is not highly ranked, that doesn't necessarily mean that its undergrad counterpart isn't good.
- If a grad program is highly ranked, the undergrad counterpart is probably at least pretty decent.
At a university, you will probably take some classes with grad students in them. Likewise, you may work with some profs who are also doing grad work/research, and at many schools your discussions or labs may be taught by grad students. So there can be some bleed from the grad program into the undergrad program.