Kashiko did try telling Sephiroth the joke, but before she could, Sephiroth just replied “It better not be one of your Sayori hanging jokes” and so she told Monika instead
Kashiko: Wanna hear another trigonometry joke?
Monika: Sure
Kashiko: So we have the angle of elevation, and the angle of Sayori
Monika: You mean angle of depression?
Kashiko: Same thing