I store images in CKRecord as CKAsset. Because of this, used a lot of disk space. Is it possible after fetch images from CKAsset, do the cleaning in at App\Library\Cashes\CloudKit\Assets?
It's not possible to clean up those files via the CloudKit API.
CloudKit will manage that space for you and should clean up downloaded assets periodically or if a low disk space notification happens. Your application shouldn't need to worry about it.