Yes.
However, use care in your selection of cups. PR will eat some plastic cups. I generally buy the generic colored plastic cups. They will still melt, but slowly enough that I can mix and pour before hand.
A couple weeks ago, I was mixing up red, white, and blue resin for some US Flag bottle stoppers. Since I didn't need much resin of each color, I didn't use the large generic cups. I used the smaller 'Solo' brand ones, instead. The Solo cups started leaking on my workbench before I had a chance to complete the pour.
Neither the Solo cups nor the generic ones would have worked as molds.