Most instructors over the years have given me mixed positive and negative comments which has been most useful (I have PPL plus bits). It helps to know where you're going wrong and where you are on the right path.
As a lecturer (and examiner) I always say how to improve. Rather than saying 'clueless' on a piece of work or presentation, I'd say 'needs a great deal of development in all areas, although you have slightly improved .... over the lesson'.