A thesis is more than just a description of your subject matter. It must take a stance that can be argued for or against. There are several different approaches which can be taken to such a subject matter. For example, you could take a sociological approach based on status anxiety grounded in the work of Veblen (Theory of the Leisure Class). You could also do a feminist analysis, arguing that the diversion of women's economic resources into image represents a form of complicity with oppression. Another approach would be based on evolutionary psychology arguing for the role of sexual attractiveness in mating. Your thesis should be based on what you consider the central explanation of why so many women spend money on beauty products.