This is an interesting question. I think it might be argued that if the plot of On Beauty were to change--as long as the characters were not also changed--the impact of the book would not be lost or altered. On the other hand, if the characters were changed, the impact of the book would be weakened or otherwise altered. By this argument, characters would emerge as more important than plot.
Smith is exploring the adjustment of adults and their offspring to the social world they find themselves in. An integral part of that adjustment comes from the fact of the parents' mixed-race marriage and the resultant mixed-race ethnicity of the children. Thus part of the exploration Smith initiates in On Beauty is what impact interracial marriages and families have on the effects of social interactions and career success and personal fulfillment.
In this light, characters are more important as they may (theoretically) go through many different conflicts and experiences--i.e., many plots--while the questions asked and the exploration undertaken may remain the same. The dominance of character in Smith's novel is indicated when Kiki buys a pie for Mrs. Kipps "after the Belsey world fell apart": Kiki's personality still dominates the plot in both action and conflict:
"I need a down home pie, do you know what I mean? Nothing French or...frilly."
Kiki laughed her big lovely laugh in the small store. ...
"See that?" said Kiki emphatically, pressing her index finger on the plastic ... in the center [of a pie] sat a red and yellow compote of sticky baked fruit. "That's what I'm talking about."