Its all in the context. I don't see Ricky Gervais as David Brent (who became Micael Scott in the American version.) He's not built up the character enough for me as Steve Carell has done in the context of the long running - very long running - USA sitcom (one of my teenagers has the box set!) Ricky Gervais is still Ricky Gervais when he's singing so it comes across perhaps as more clever-clever-cruel than I think he intends. And even more so when the song is available commercially in a collection of similar songs, so that people might ... sing along at a party?
I do see what he was intending, choosing the predictably unacceptable things to say about people with disabling conditions and letting David Brent say them grotesquely outloud cos that what his character does. Trouble is, most people do get that it's unacceptable to make fun of those who are disabled by a health condition or illness. I hope so anyway. But people don't get what's wrong with saying to ME sufferers, 'Oh you have ME. Well, I'm tired too. Everyone's tired'. People with ME hear it all the time. Not maybe as directly as that but people do tell me very very very often how tired they are ... non verbally, by actions, facial expression, text and email, verbally, face to face, behind my back but within earshot. That and many other pokes chip away at ME's credibility as a real illness.
It's easier to have a poke at the illness than to take time to find out that it's a very serious, life destroying illness that most in society don't understand unless they're touched by it. Mention MS, Dementia, Cancer and people get the life changing enormity of these - and don't, I think, write 'funny' songs about them. ME isn't like other illnesses that dis-able; it's a stigmatised illness that people think they can take a poke at. I'm afraid the Disableds Song gives the message that it's okay to tell people with ME that they're merely tired. It's what people already say.
In context it's David Brent's awkward, clumsy verbal and social ineptitude; out of context it is Ricky Gervais being cruel which I don't believe he is. I read the lyrics of the song and referred to him in a tweet as the Katie Hopkins of comedy. I don't think he is. It's all in the context. Sorry, Ricky, if I've got this wrong and if cruel is what you were trying for!