I installed Zorin 17 Core as a Guest on Windows 10 host, using VB. Two odd things happen. First, Zorin flickers a lot, especially when I launch a browser, such as Chrome. The other problem is that my cursor becomes a dark square. My System settings are as follows: 4096 MB Base Memory, 2 CPUs , Execution Cap: 100%. My Display settings: Video Memory 128 MB, Monitor Count 1, Scale Factor 100%.
Have you tried running the VM with the DE Zorin on xorg? Zorin 17 runs on Wayland by default. When you get to login screen of VM, select your user, and a cog appears bottom right. Change from Zorin to Zorin on xorg and then login. See if this solves the issue.
I finally got the shared folder capability to work in VMware Workstation, so I don't think I'll mess around with VirtualBox anymore. VMware displays of the Zorin client doesn't flicker.