The word is practically synonymous with Paris Hilton but does not, in my humble opinion, cover superficially similar inviduals like Lindsey Lohan or indeed any other visibly stupid and annoying celebrity. I realise I am going against the tide here but what humour there is to be derived from the use of the word "celebutard" is intimately connected to it's development from the word "celebutante" and not it's more apparent relationship to the words "celebrity" and "retard".
"Celebutard" would not have been coined if "celebutante" had not existed first, the former is a play on the latter as far as I am concerned. How else to explain the connecting "-u-" when any other vowel (or none at all e.g "celebritard") would have done just as well? There is no "u" in either "celebrity" or "retard", there is a "u" however, in "celebutante".
It follows that someone like Hilton could be described as a "celebutard" since she is all three things, an erstwhile débutante, a celebrity and... ahem. Lindsey Lohan, on the other hand, doesn't actually fit that deb-y, upper-class "It girl" bracket. I guess people find it funny to describe her as a "celebutard" where they wouldn't have described her as a "celebutante" but for me, it's a pretty weak gag.
Whatever, I feel like King Canute on this one but I had to go on the record.