"Jimmy Olsen jokes are pretty much gonna be lost on you, huh?"
I chose the above quote for the bizarre irony of it: the reference that gives this episode its title is likely to be lost on a substantial chunk of its audience. Anyway.
The contrast between this episode and the previous one is really quite striking: While "Helpless" was incredibly dark despite featuring a lone vampire who posed a threat only to Buffy and her mother, in "The Zeppo," a struggle of apocalyptic proportions is taking place but comedy is the order of the day. And "The Zeppo" is just as successful in its comedy as "Helpless" is in its dark drama.
The juxtaposition between the two different storylines in this episode is great, as the writers poke fun at their own material by providing us with an alternative window into the apocalyptic drama of the program. Meanwhile, Xander carries an entire adventure on his own, complete with violence, car chases, sex, humor, and a moment of extreme bravery and strength at the very end.
The question the episode raises, as to what Xander's function is in the evil-fighting missions of the Scooby Gang, is certainly one that's been on many viewers' minds up to this point. But the episode argues that the question is really sort of a moot point: far more interesting is the role he plays in the show itself. By extension, it demonstrates that what really matters in terms of how well they work in the show is who these people are, not what powers they have. And you'll hear no dissenting opinion from me.