Everyone gets mad about Buffy getting beaten in this episode, but that was kind of the prevailing message of it. Buffy was a mess, and not just with her slaying. She was totally and completely overwhelmed by college, by leaving home, by Willow and seemingly everyone else being ready for it all while she felt left behind, and most of all, by Angel not being there. All of that did a huge number on her, and Sunday was in the right place at the right time to take advantage of it. That happens. Even the best champions get knocked down once in a while when the circumstances align in the right (or wrong) way.
I don't think that Sunday or the episode on the whole made Buffy look weak at all (after all, she does get Sunday in the end). It was trying to convey a theme about what college is like, and I though it hit a home run.