Lufthansa runs an express coach from MUC to Nurnberg.
https://www.lufthansa.com/ch/en/lufthansa-express-bus
Otherwise: Just buy a ticket MUC - Nurnberg. You can buy a flexpreis ticket in advance. Such a ticket is good for any train on the route, so you can then just proceed by taking the next train that comes along. Such tickets do not automatically come with reservations but these are not required.
What you could do is buy a flexpreis ticket, and a reservation for the train you are most likely going to be able to take. If you miss that one because of a delay on your flight you still have your ticket. If you get to the main station earlier than expected and see that there is an earlier train you can jsut take that, and see if there are seats...