Katherine Houghton Thought Joanna was a Ninny Too
Katherine has said that she did not like the way Kramer wanted her to play Joanna, all wide-eyed and unaware and that she too could not see how Poitier's character could become interested in a person with so little experience, even if she was pretty. If the character of John was a world traveler, as depicted, he would have met many women of all colors who would have had greater intellect and as goodlooking or better and who would not have been hobbled by race prejudice (since, in a lot of other countries, interracial marriage was not an issue.) And as a goodlooking, famous doctor, John (Sidney) was a catch.
The only thing I can figure, as another poster proposed, is that Kramer did not want the Joanna character to become involved in the dispute, so her joyful, blitheful attitude was the only way to keep the character from becoming involved. If you want, just imagine that John was hot for her and dazed at her total lack of seeing any problem with their falling in love.
Give Miss Houghton credit for taking the part. She received a lot of death threats for it.