In my non-expert opinion, both Dennis and Carl are right. Good comedy (IMHO) -- of this sort -- parodies/ridicules/caricatures the prevalent (powerful) mores, culture, attitude, etc. Bad comedy falls back to the use of stereotypes for easy laughs from a homogeneous majority. Borat, like The Simpsons (with its Indian stereotypical character) does some of each: expose the hypocrisy, illogic and other silliness of popular opinion (in the USA), but also use (even if incorrect) easy devices such as caricaturing foreigners (as Guardian points out) to seal their success (why is Borat more successful for Western audiences than the original Ali G?). The result, at least to me, is ambiguous -- I have met few who talk about the social commentary that is a large part of The Simpsons, but quite a lot of clowns who think they are being clever and funny with an "Appu" imitation every time they come across a bunch of Indians (either leaving them puzzled -- best case -- or cringing -- worst case). I think the Guardian (who may be pompous etc -- I don't know) is quite right about that.
--ravi