The issue here is that making light of racism, even in something as inconsequential as a Slap thread, is still a dangerous thing to do. It undermines the gravity of racism and starts to normalize it: "this isn't so bad, look we can still have a few laughs about it!" Which, if you experience(d) racism, can be extremely frustrating to say the least and it undermines progress. In a broad sense it's the same thing as JJ using Nazi ephemera. Whether or not he's serious about it and/or it was when he was younger, they are still symbols of hate. And to put it a certain way, not everyone's going to get the joke.