Use this command to connect to your Marlin-based 3D printer:
picocom -b 115200 /dev/ttyUSB0 --imap lfcrlf --echo
This command might also work for firmwares other than Marlin.
By default, picocom uses character maps that cause the newlines not to be shown correctly. --imap lfcrlf maps line feeds sent by the printer to CR + LF on the terminal. --echo enables local echo, enabling you to see what you are typing.