Thomas Aquinas saw a connection between the metaphysical existence of God and the values of morality in the sense that they are both grounded within His existence.

In other words, Aquinas saw God as the basis for and to all we experience. From the theistic perspective, my question is if moral subjectivism is true, it seems to be self defeating as the concept of truth is fundamentally underwhelmed.

There are plenty of atheistic and some theistic philosophers who see morality as independent of God. I wouldn't necessarily agree with them, but it's just a thought