I'm applying for Astronomy/Astrophysics PhD programs in the U.S. My undergrad GPA at UT Austin is 3.1/4, but my Master's GPA with all graduate-level classes is 3.85/4 so far, which is a very big difference. Should I mention this in my SOP?
No. They already have your transcripts. The purpose of your SOP is to answer the question, "What is your purpose is wanting to pursue a PhD?" It should be future-oriented, explaining your goals and what they should expect you'll do if they admit you. Your SOP is not the place to explain how you've dreamed of this since you first looked through a telescope as a child, nor is it the place to explain how much better you really are than your grades might suggest. Stay on point. Tell them about your plans for the future.