Just speaking as someone who has difficulty reading tone irl, let alone through text, yeah there needs to be more indicators that this is a joke. I did not know this was until looking through the replies (and also looking through the replies, I clearly wasn't the only one). I don't think it's necessarily "Karen behavior" to ask that a joke be made clear, and actively mocking the idea that someone doesn't understand a joke is immature at best and downright ableist at worst.
I think that when it comes to posting a joke like this, you have to think "what would happen if this post inevitably reaches outside of my target audience, how should I react to that?" and if your reaction is to insinuate that anyone who doesn't get it is dumb because it's "so obvious," maybe you shouldn't post it (this isn't necessarily directed towards the moderator, as I haven't see them getting defensive to the point of mockery in this thread).