I want to laugh in his face
Why do you want to laugh in someone's face?
I'm going to tell a joke about a mentally disabled person
You shouldn't make jokes at the expense of disabled people. Mocking someone is hurtful and then to laugh in the face  really cruel. Have you thought about just trying to be nice to them?
I didn't think it was the best thing to do. I'm going to laugh in his face.
It's not okay to laugh at anyone, especially a disabled person.  Instead, you should practice being kind and considerate of the feelings of others.