He's right, Childermass does not get the joke. One might even say it passes so far over his head that it's in orbit (which is another joke he would never get). Yeah, he frowns briefly, but he lets it pass. Joker is a strange man and not even the strangest, so he gets a lot of leeway here.

Oh, but he will grimace noticeably at the very thought of the Doctor and Strange meeting.

"Now there's something I'll be stopping at any cost. I'm not sure the carnival would survive those two teaming up, especially for a magic act..."