Never really found him that funny TBH, but each to their own.
I do recall when he became embroiled in this though:
http://news.bbc.co.uk/1/hi/entertainment/598586.stm
Odd given that one act I recall Dexter doing was that of a Nigerian immigrant.
Isn't the Ali G thing no different to the stereotypes so brilliantly mimmicked by the Goodness Gracious Me cast when they 'go for an English'?