What is powerful about these two women is that they both clearly have the ability to pull off both looks if they want to; what is unfortunate is that neither of them do so very often (at least in the public eye).
While being a mix of sexy and cute is better than being only one or the other, the ideal is having the ability to be completely one or the other at will.
You just need to take a step outside yourself for a moment, and acknowledge that there is no real impediment to you pulling off "sexy" any more than there is for the thousands of girls who regularly do so.
While you might have a mental block against the idea of you being "sexy," I challenge you to list concrete things that make you different from the women that dress and act that way - and I am not only talking about celebrities like Megan Fox and Taylor Swift.
So don't start buying hooker outfits arbitrarily, assuming that they will make you sexy; make sure it works with your natural look (notice I did not say personality).