Mr. and Mrs. Crosby are American caricatures through and through. Mr. Crosby owns a bicycle factory that he plans to move to San Lorenzo because the Chicago workers "sit around trying to figure out new ways for everybody to be happy" (42.6); Hazel is obsessed with Hoosiers.
And they're both shining examples of granfalloons. (We think it sounds like "buffoon" for a reason.)
In Bokononism, a granfalloon is a fake karass, a "seeming team that [is] meaningless in terms of the ways God gets things done" (42.42). Since a "karass ignores national, institutional, occupational, familial, and class boundaries," we can assume that all these things are granfallons (2.2).
So—your friendly group of football fans? A granfalloon. The group of people at your 5-year high school reunion? A granfalloon. Your meeting of American ex-pats in Paris? Another granfalloon. These are all groups based around a completely superficial identification. They don't have anything real in common; instead, they're constructed around made-up social categories.
Mr. and Mrs. Crosby remind us why granfalloons don't fit in with humanist philosophy. Both characters are friendly and supportive, but only to people who share certain superficial traits. For example, Mr. Crosby supports the San Lorenzan government because they seem to express his American capitalist ideals, despite the island's insane poverty level. In the same vein, Mrs. Crosby loves all Hoosiers instantly. She also finds San Lorenzo welcoming because she thinks everyone on the island is a Christian (62.10).
Sure, neither of them is a completely horrible person. But they're portrayed as, well, a little dim-witted. They can't let go of their granfalloons even when the world is ending: Mrs. Crosby ends the novel by stitching together an American flag.