'Cute' nowadays just tends to mean small and attractive. The 'button' phrase is possibly just a variant of 'cute as a bug's ear', since both begin with a 'bu' sound and both are small, daintily-fashioned things. If you click http://www.phrases.or.../41/messages/652.html that will link you to a web-page with some relevant information. However, there is no certain source for many such comparisons.