The likely answer is that, without actually getting into a copyright issue, the filmmakers did just enough to make this seem like The Joker without actually calling him The Joker.
Does The Joker have a cameo in Deadpool 2?
It’s not necessarily surprising to find out that the Deadpool sequel has some cameos and unexpected appearances in it.
But this is definitely something to keep your eyes peeled for on your next viewing of Deadpool 2.