This seems to be a common misunderstanding of what was actually going on. On top of that, Republican activists are clearly trying to push this line, even if they do understand the reality of the situation.
And the reality is this: the committee was not simply digging into the minutiae of Kavanaugh's student years for their own sake. Rather, the committee was doing so specifically because it has a mandatory requirement to try to ascertain the veracity of certain claims made against Kavanaugh, which date from around that time. Those claims, if verifiable, would constitute not only a potential criminal offence, but would also necessarily disqualify Kavanaugh from taking up a position on the Supreme Court.
With that in mind, in order to try to assess the veracity of the claims against Kavanaugh - and specifically Dr Ford's claim - the committee (correctly and appropriately) was trying to assess Kavanaugh's general character and actions from around that time. Only an idiot or a politically-motivated operative would disagree that it's irrelevant to the committee's investigation for them to question Kavanaugh on things like his (outrageous and self-character-assassinating) high school yearbook entry, his drinking habits from around that time of his life, and his specific activities from around the potential time period of the alleged assault.
So yes, if taken as a means to an end on their own, it would indeed have been inappropriate for the committee to have been delving into these sorts of issues. But in the context of investigating very serious - and disqualifying - accusations, they were/are entirely appropriate and fair.