Better don't go all the way to Mainz Hbf but alight from S8 at Mainz Römisches Theater, the first station after the train has crossed the Rhine. It is closer to the oldtown.
Check out this article which describes a walking tour through Mainz: https:/…Walk.Mainz.In.One.Day.html It can be done in 3 hours.
Yes, the major sights are all within easy walking distance from station Römisches Theater.
Another way could be get off at Mainz Hfb and take a tram 50,51,52 from Hauptbahnhof to Schillerplatz 2 stops and start from there. After-all i have a day pass to travel Mainz.Do you think that is a better than the stop at Mainz Römisches Theater ?
You can do that as well and then walk up to St Stephan church.
I like Oli6's advice. It's what I would have told you. There's a wonderful Baroque church in the pedestrian area. The Cathedral is super. It it's raining and you are medium well dressed, go to the Hyatt and get a glass of local wine or cup of coffee and watch the Rhine flow by. Mainz is a lovely town. Lots of student, a provincial capital with Roman roots.