Gervais always takes the same approach to marginalised groups - in The Office he simultaneously earns respect for casting disabled actors to play disabled characters, at a time when that didn't always happen, and then plays both sides, when it comes to using them as a comedic device. The same with Extras and Francesca Martinez's casting - he again laughs at her disability, yet writes her as a character, rather than just a plot device. I think he might just be one of those characters who genuinely believes that there are no limits when it comes to comedy. If you look at the guy who is he is responding to in that Tweet, he has an equally sardonic tone about nearly everything he posts, and Gervais is just responding in kind. I might be giving him too much credit, but I honestly don't believe he is a terrible person, just one who thinks that anyone is open to mocking.