Someone who has long been against some civil right or group- vocally- will then turn and pretend that "it's no big deal".
"So what? He's gay. I don't care. I'm not prejudiced. Why does he have to make a big deal about being gay?"
"I'm color blind. What's the big deal? Why do so many black people have to act like being black is something now we need to notice when they spent all that time telling us that they wanted to be treated like everyone else?"
"I'm not sexist. Okay, women are equal. So why do they keep expecting me to give them extra rights, like this maternity leave? If they want to work and have babies too, why should they expect special rights?"
"So what?"== it doesn't just deny everything said before in denial of the person's rights, but also dismisses the very important and interesting experience this person has lived. Lil Nas has a fascinating experience to share, and also an important decision to come out at this moment in his career. But Kevin Hart says, 'So what?' to cut that off, to say that this experience is "no big deal", and that even mentioning it is expecting more than he has a right to expect.
That said, we're wishing a quick recovery for Mr. Hart and his companions after that crash (in what would have been my dream car in the 70s, a blue 'Cuda).