No it won’t be unfortunately. I posted about this a while ago, can’t remember the exact details although I did look it up at the time. RB Leipzig tickets are valid only on regional transport (which excludes the Deutsch Bahn intercity trains) in Saxony. So it will cover the tram/bus/city train etc on match day, but not getting to Leipzig from further afield in Germany.
You’re right about the mask though, you are supposed to wear one on Deutsch Bahn trains. Albeit, a fair few people don’t bother these days compared to very high levels of compliance in the earlier days of covid.