Is a prominent cisterna magna anything to worry about?

It appears that it's of concern in the fetus because of potential developmental problems. In an asymptomatic adult it is considered a normal variant which doesn't warrant further studies or follow-up according to radiologists.

You describe a congenital variant of no clinical significance in a healthy 32-year-old. Assuming there is no additional problem such as a Chiari malformation, would not see the need for further evaluation.