There's a couple things that seem off here.
Firstly, a mail carrier could not get in trouble for saying any of the things that have been said here. He could be attending rallies and campaigning for Biden and also publicly saying that he believed DeJoy and Trump were rigging the election and that would not violate the Hatch Act:
In the past it has been standard practice to deliver mail-in ballots regardless of whether or not they have postage.
Also, although Trump did basically admit to trying to sabotage mail-in ballots to rig the election, most of the suspicious changes that were taking place were actually decisions made before COVID19 and before Louis DeJoy took office as Postmaster General.
https://www.linkedin.com/pulse/status-mail-delivery-from-people-who-watch-living-dave-lewis
https://gen.medium.com/stop-panicking-about-the-post-office-8bcd689b9601
