Have you noticed that for bluepill, fo serial monitor you need a USB to serial adapter connected to pins A9 and A10? Serial over onboard USB by default doesn’t work (I have not yet found a way to get it working at least). Maybe you have RX and TX pins accidentally switched?
I suspect that the PlatformIO serial monitor on Atom should just work but be aware that the preferred editor for PlatformIO is Visual Studio Code (IIRC last time I checked Atom was not even listed as supported editor anymore).
I don’t know what OS you are using but for Windows Termite is a handy and easy to use terminal program that can also show the serial output (this will allow you to verify if serial monitor in Atom is possibly the issue).