I should say that is possible to add a new user to an ubuntu server through VMware Tools in post provision. But let me tell you, with Cloud-Init this is by far more simple.
With cloud-init you just need to add a couple of lines in the Cloud Template.
With only vmware tools, you need a subscription to vRealize Orchestrator workflow, use an Action with PowerShell and you also need vCenter Credentials + Ubuntu credentials to execute the commands.