Apparently, Kali Linux 0.6 Raspberry Pi 3 w/ Nexmon contains older packages. Once issuing the command apt-get update && apt-get -y upgrade && apt-get -y dist-upgrade tightvncserver gets replaced by tigervncserver. Thus, TightVNC isn't part of this image of Kali Linux. TigerVNC is a fork that is supported by the standard Kali Linux repositories.