I have mitral valve prolapse and a mild heart murmur, is this common and sort of normal?

Can be. Mitral valve prolapse is most often associated with some degree of mitral regurgitation ("leaking") which causes a murmur. The severity ranges from mild and insignificant to severe. Depending on the severity and your age, it may never, ever cause you any symptoms or problems. In the rare cases in which it is very severe, the valve can be repaired and life goes on normally again.
Probably so. About 10% of the population has some minor, insignificant form of mitral valve prolapse, but it does not generally affect their lifestyle. Most of the time, there are no (or few) symptoms, and treatment is not needed.