Welcome in the SBB community. This is a users forum, and as such you will not get an official answer from SBB here. If you want to talk to someone from SBB about this you need to use the official channels for that. The document you received with the fine should mention where you can contest it.
That said. You were in the wrong. On those trains the 1st class is marked with a yellow stripe on the outside, with the numbers 1 and 2 near the doors, and the head rest of each seat is red. So it is not the case that one could easily by accident sit in 1st class. As a regular traveller you are not expected to make these kinds of mistakes.
It has become a regular issue that people sit with 2nd class tickets in the 1st class, and then, if there is no control they have enjoyed 1st class comfort at 2nd class prices. The expectation has grown amongst passengers that if they are caught with a 2nd class ticket in 1st they can just say “oh, sorry, I didn’t know” and move to second class.
SBB apparently has recently decided to no longer tolerate this. So sitting in 1st with 2nd class tickets now has consequences. I doubt that you will be able to get the fine waived.