2018 MacBook Pro + Dell U2718Q, single cable for display and USB data
No.

Technically speaking, you have a Thunderbolt 3 Type C port.  "C" is the physical charastic of the port, not the USB specification.  Being that it's Thunderbolt 3, its carrying several signals:


PCIe
USB 3.1
DisplayPort
Power


Your "end" device is a monitor that has several different inputs


HDMI
DispayPort (DP)
Mini DisplayPort (mdP)
USB


When you go from your MacBook's TB 3 port to the video in, you're not passing the USB signal; this is why you need a second cable for the USB functionality.  If this was a monitor that accepted Thunderbolt as an input, you could do this with one cable.