Yes, remembering all the single feedbacks can quickly become an unbereble burden (depending on the implementation). Depending on your implementation, saving the current average of all feedbacks (or sum) and number of them is a fine solution (as long as the single values are not needed).
Do note that to allow the user to choose y in
"GetMostRated(x,y): returns the content with highest rating for feedback category y (or highest average of all ratings if y is not specified)."
you do need more than one global feedback average.
