Un-normalize having to apologize for not getting the joke. Eradicate having to insult your own intelligence just because you find someone else to be unclear. You have nothing to be sorry for. You've done nothing wrong. People who call you dumb or mock you, or even just expect you to apologize for the way your brain works, are ableist shits who don't deserve you.
“Why are you so gullible?”
“It was a joke, calm down, why can’t you see that?”
“Why would you believe me when I was obviously joking? (With my serious tone on.)”
Oh, I don’t know, maybe it’s the autism?
If you’re trying not to be ableist, a little tip: try not shaming people for believing you, if you say something in a way that approximates seriousness? Autistic and other neurodivergent people sometimes have difficulty with tone and discerning sarcasm.
And if somebody has trusted you enough that they believe you mean what you say and are willing to consider it themselves… it might be just a little nasty to laugh in their face for offering you that trust. Mocking people for taking you seriously, when the whole point of your joke was that you were saying something you didn’t mean in a serious way as though you did mean it… isn’t really a nice thing to do.