Television & Movies

Here's the thing, by mocking said racist stereotypes, he's still also perpetuating them and giving cover for genuine racists. Here's an article about ironic racism, also known as hipster racism:

Hipster racism ... "the idea that something ordinarily offensive or prejudiced is miraculously transformed into something clever, funny and socially relevant, by the assertion that said ordinarily offensive thing is ironic or satirical." ... "those participating in acts of hipster racism understand those acts as racist when practiced by others, but rationalize their own racist performances through a presumed exceptionalism."

You have the same thing with sexism:

Hipster sexism, also known as ... ironic sexism, is defined ... as "the objectification of women but in a manner that uses mockery, quotation marks, and paradox". It is a form of self-aware sexism that is deemed acceptable given that its perpetrators are conscious of the inherent sexism and objectification of women ... It is rooted in the idea that sexism is an outdated and archaic institution which people do not engage in anymore, thereby making the demonstration of sexism seem satirical and ironic. ... may be presented with derision and expressed as harmless. ... Hipster sexism relates to postfeminism in that it downplays sexism at large by casually normalizing it on the basis that sexism has been eradicated and thus is not appropriate for serious consideration or discussion.

Of course, neither sexism or racism have been eradicated. Far from it. In fact the far right have used the irony defence to mainstream racist and fascist ideology:

Marwick points to another guide to the “alt-right”, published last on Andrew Anglin’s prominent Nazi site, the Daily Stormer, which credited “troll culture” with bringing about “non-ironic Nazism masquerading as ironic Nazism”: "Irony allows people to strategically distance themselves from the very real commitment to white supremacist values that many of these forums have." ... When Richard Spencer led a fascist salute to Donald Trump at his National Policy Insitute conference in the wake of Trump’s win, he said it was done in “a spirit of irony and exuberance”. ... "Fascism is more or less a social taboo. Humour is one of the ways that they can put forward their positions" ... More generally, every “ironic” repetition of far-right ideals contributes to a climate in which racism, misogyny, or Islamophobia is normalised. ... deployed by the far right in chipping away at whatever prohibitions have existed around publicly adopting far-right politics.

For example, the reddit community /r/TheDonald, was originally an ironic sub. But a lot of people didn't get the joke. So much so that before it was banned, it became the biggest pro-Trump community online, and helped get Trump elected.

Similarly, Ricky Gervais also makes similar claims to being ironic when he makes transphobic jokes, but doesn't seem to mind his rise in popularity with the far right one bit.

TLDR: it is possible to understand Ali G is satirical, and still be critical of the character normalising racist stereotypes and giving genuine racists cover.

e: made some cuts for brevity.