The amount of time OP is planning to spend in Mainz is pretty much perfect. A few years ago I had about two nights (or was it 3, I can't recall) in FRA before meeting up with a group, and I decided to spend them in Mainz for a change of scenery. It was pleasant, but I recall it was a bit too long whereas one day and night is sufficient to get a good feel for the place.
Also, getting to/from Mainz is as fast and simple as one could imagine.