Yes —
usually yes. The large
side physical pads / shell tabs of the USB-C connector
J6 are normally connected to
GND/shield, so they are a good place to solder the converter negative
if your board connects them to GND.
Also: soldering to the side tabs can take more heat because they are large metal/mechanical pads. Make sure the joint is solid, and add strain relief for the battery/converter wires so they don’t rip the USB-C connector off the board.