There is a lot of moisture released with the apples cooking. To have a crisp topping, the moisture needs to be removed. Many recipes will try to absorb that humidity with paper towels or a cloth towel over the top of the crock pot under the lid. I have some safety concerns about that, and it doesn't work well. For me, venting the humidity near the end of cooking works better without the fussy or safety concerns. If you don't address the moisture adequately, the topping will not be crisp.