We are all blinded by looks
One key idea that John Hughes has developed from the film is that we are all blinded by looks. This idea is best brought out by how Andrew's impression of Allison changed after her make-over. He clearly thinks highly of appearance, as Allison was still the same girl on the inside, but she looked different. This shows us how we put appearance before personality. This is a flaw in most humans and Andrew was no exception. Andrew fell for the pretty new Allison, not the bizarre looking basket case she was before Claire's little experiment.