The poet John Keats famously said
“Beauty is truth, truth beauty,”Mthat is all
Ye know on earth, and all ye need to know.
One way to approach truth and beauty in the novel would be to examine the character of Dona Yolanda Laura Maria Rochet de la Torre (Mamita). The girls' grandmother is willing to trade truth for beauty-- she forces her husband to accept a bogus diplomatic post because she likes shopping in New York.
Another point of entry for the topic would be the difference between beauty standards in the US and in the Domincan Republic. Suddenly skin color is an issue, and beauty does not seem to be universal. This shows how truth and beauty are separated by culture. Culture clash, which includes the concepts of beauty, is a major theme in the novel.