We went for Christmas a few years ago and it was lovely. We went to Agadir as the more northerly areas like Casablanca wont have the climate at that time.
However people we met who went the week before us said that it had been cool and wet so we were lucky.
Have been going to Morrocco for almost 30 years. For the past 20 or so to Agadir. Great beach, friendly natives, delicious food, exquisite culture and best climate on earth. High season sees hazes occasionally blotting out the sun. If you drive up the coast to Essuira you'll get great breakers. Ballybunnion with sun! As regards winter hols .... Christmas Day is usually 20* to 25*. Not a place for all-night ravers.