Does the orientation of the mounted DWM1001-DEV anchor matter?

Can the anchor be mounted on its side with the USB port facing downwards towards the ground?

It may work. But the DWM1001 antenna is vertically polarised, meaning that the module is intended to be positioned vertically upright when used in an RTLS system. An omnidirectional radiation pattern is seen in the XZ plane when observed by another antenna which is also vertically polarised.