Yes, that's why I wrote 'kinda'... I don't know how can they manage to remote control the Predator's mandibles and mouth to mimic a so-called 'smiling'. What the mandibles can do are opening and closing. And the pussy mouth...nothing.
I think if the Predator laughs instead of smiling, it's better solution. We'll see in the theater.
Anyway, I think he is smiling because of the gun jam.
Or a more sexual context...he is smiling because Casey is naked.