There is really nothing much to think about here. If we take Oscar Wilde's definition of "celebrity" as "someone who is famous for being famous": as apt, then Ms. Hilton is a perfect celebrity.
She is reasonably pretty, but nothing that some bleach, a little silicon and a bit of dieting couldn't produce in almost any supermarket checkout lady. So her look is achievable or at least aspirational for most girls. She is seriously rich, which means she can have anything (anyone) that money can buy. So she works as an object of fantasy.
What does she do that people could fantasise about? As far as I can see, her activities are limited to acting like a jerk and balling a lot. Those are things that most people never do as much of as they would like to. She appears to be talentless and stupid. I say appears, because if she were intelligent, she would be smart to hide it. Any visible talent or intelligence would reduce her possibilities as a blank, fantasy sheet, ready to be scrawled upon by all and sundry.
Scarlett Johansson, for example, is truly an unforgettable beauty, possessed of obvious intelligence and a formidable talent: most of humanity, if they ever even dreamed of getting it on with Ms. Johansson would wake up and apologize. But it just might be possible for a patient onanist to imagine that Paris Hilton could wake up some afternoon and say to herself, "today I've just got to do it with an overweight, out of shape, pimply, high school dropout!". A perfect fantasy object... a truly humantiarian gift to all too human humanity.
Then of course the media finds that she provides them with content that isn't drenched in blood and hate, and that she allows a reasonably literate person, who reads things written to be read to feel superior. And that of course, for all of the above, Paris Hilton draws readers. She is a perfect example of the Taoist ideal of "Wu Wei", not doing anything, all is done.
Full disclosure: this post itself is an experiment. I'm interested in following it in my trackers and seeing if the tags draw more that normal hits on a slow Monday. Testing the "Paris Hilton effect" its called. I've tried to make the experiment as entertaining as I could. I hope faithful readers of the blog aren't horribly offended. DS