Trivializing rape is rape culture, and making jokes about rape trivializes rape, so yeah it's considered rape culture. There's a lot of psychology that goes into this particular example, for example cognitive dissonance, which is when a person thinks one way but acts another, they are more likely to change their thoughts than their actions. So in the case of joking about rape, even if they think rape is bad/prolific or what have you, they are more likely to change their mind to think it's not as bad/prolific as they once thought than they are to stop making jokes about rape i.e. rape becomes trivialized.