BEST BOY -- At the K-B Cerberus 1 and K-B Baronet West 2.

In our popular drama, mental retardation is usually seen either as funny (as in "Being There") or as sweet and magical (as in "Charlie and Algernon"). It would be helpful if the presentation of an Academy Award to "Best Boy," a documentary on the subject, means that filmmakers will be exposed to the problem in a realistic way.

But "Best Boy" is not, itself, likely to be a popular success. It's a documentary in the simplest sense of being a lot of home-movie reality without the structure to give it dramatic impact.

Over a three-year period, Ira Wohl filmed scenes in the lives of his 52-year-old retarded cousin, Philip, and Philip's parents, Pearl and Max. Wohl was also the catalyst of change in Philip's life during that time, insisting that Philip's elderly parents stop sheltering him at home and send him out to be trained for independent living after their deaths. By the time the filming was completed, Max was dead and Philip living in a house with other retarded adults; Pearl died shortly afterward.

They are appealing people. Philip is sweet, not because he is retarded but because he resembles his endearing and exasperating mother, who says all the wrong things ("You don't want to stay home with Mommy no more?" she asks as Philip takes his first important steps toward caring for himself), but whose face all the while radiates such love as to be the sunshine for his growth.

Daffy Pearl and her dour husband, Max, might have been quite a comedy team if they had lived. She has a sort of Gracie Allen vitality that serves as a foil to his grousing. They can make a funny routine out of such dialogue as: "I'm hungry." "So eat." "I'm eating."

But there are endless scenes in which she coyly protests against being photographed in her housedress, or he sits stubbornly thinking about his own health problems. Philip's progress is not shown in a way that lets the audience participate in the struggle. It's the stuff of realistic drama, but not a real drama.