I have diverticulitis and am currently on a low residue diet. Is coconut milk ok?

Yes. If you have diverticulitis, with pain and fever stay on a low fiber diet for 4-6 weeks to allow healing. If you merely have diverticulosis, then a high fiber diet is recommended and include 80-100 ounces of non cafeinated, non alcoholic fluid per day and this can include coconut milk.
Has good electrolyte. Pure coconut water has good electrolytes, coconut milk has additional fats and carbohydrates, which gives calories, regarding diverticulitis is a disease of western world, infrequently ( rarely ) seen in the developing world where bulk high fiber less processed food is consumed.