THE blue-eyed, fair-skinned homemaker on cookbooks and cake-mix packages will soon be replaced by a new, multiethnic Betty Crocker, who will better reflect the growing number of minorities in the United States. General Mills Inc. has asked women to send in mugshots and an essay describing how they embody company ideals, including a love of cooking and baking. It will then select 75 photos and digitally ''morph'' them into a single Betty. The company wants to revitalize its products, and updating Betty might turn out to be a good way to do it. It might also be taking its cue from Quaker Oats, which turned Aunt Jemima into a slimmer and less offensive version of her former self. A few questions, though. Are the ones buying Betty Crocker instant cake mixes and Hamburger Helper really the women who ''love cooking and baking''? Aren't they rather the working women and men who have little time for it? And while Betty gets a new look, is she holding on to certain ideals of American women - being an enthusiastic and creative cook - that also need updating? One hypothesis: Yes, Betty Crocker consumers are looking for simplicity, but they haven't quite rejected the concept of baking from scratch and all that that implies. Betty, they hope, can offer them the best of both worlds.