From the US Office of Special Counsel, the agency that enforces the Hatch Act:
A U.S. Postal Service employee in Colorado displayed a “Make America Great Again” – the slogan of Donald Trump’s presidential campaign – behind the desk at the post office. OSC contacted the postmaster of that facility and advised that the Hatch Act prohibited display of the sign. OSC confirmed that the sign was immediately removed from the post office.
Source: Office of Special Counsel