I'm going to make a joke about a friends mom
Why do you want to do that?
Because she teases me and I'm going to do something about it.
She is wrong to tease you in that way. You will have to stand up for yourself even though she is older. Let her know you don't appreciate what she is doing.
I can do whatever the hell I want and she can deal with it
That's not a good thing to do. You shouldn't make fun of people, and she needs to know that. Being an adult she should know better.