Whether the fine, which is actually rather a surcharge, is justified, can only be answered by the responsible department dealing with travel without tickets. You have to reach out to the phone number which is mentioned in the lower part of the receipt that you have received for travelling without a ticket. What concerns travel on an SBB service, see: Help with travel without a valid ticket or with only a partly valid ticket | SBB help & contact
As I mentioned previously, we are unable to judge your case. From the tariff (Übersicht der Tarife und Vorschriften - Alliance SwissPass) perspective, the fine is justified as you are clearly required to check in before boarding a public transport service, and this is valid no matter whether you have travelled on a day pass or not. You must be able to show a QR code during ticket check, which acts as justification that you have checked-in successfully.