Richard Herring called him on it, and said it very succinctly here
"Ricky Gervais is new to Twitter and seems to have spent the first couple of weeks mainly posting pictures of himself pulling the kind of faces that school children pull to parody the disabled and calling people 'mongs'.
"I think many comics are guilty of using [the word "mong"] as convenient and humourless punchlines. I don't think any of them would do the same with the word 'n*er' or 'p**i' but they're happy to use 'mong' or 'retard' as a means of getting a laugh. And audiences will laugh at those words too and rarely even complain about them.
"If the words are upsetting some people and perpetuating a stereotype, isn't it more noble and thoughtful to just admit you might have made a mistake and stop?"