What's your opinion on running Windows Server in a VM?

(This isn't for any important or production application)

Can't offer too much insight there. I've only ever run it as the primary boot OS. 

If you're running it within a Windows install I can't see too much point in it. Would make more sense to have windows server as the main OS and run a VM for regular windows 

I don't see what purpose a virtualized server would have. If you need to share disks across devices, just use Storage Spaces in any consumer Windows OS. If you're trying to test out Windows Server in a VM, just pick something like CentOS, FreeNAS, or UnRAID since they're generally less expensive, have a few more abilities, and come with more "pro" features such as a broader reach for different systems/target OS's. 

I'm going to be running PROXMOX which will host at least 12 linux VM's and I figured I'd experiment with Windows server while I'm at it.

@ARikozuM

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generally less expensive

The college I attend and CIS being my major gave us access to quite a number of expensive pieces of software through the Microsoft Imagine program. One such piece I was given was a Product Key for Windows Server 2016 Datacenter Edition which googling is a damn expensive version of windows.

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just pick something like CentOS, FreeNAS, or UnRAID

I currently have a FreeNAS server. The plan is to bridge one 10Gbe network adapter for all VMs on the PROXMOX server and enable the NFS service for the 10Gbe NIC in FreeNAS. From there I can run Windows Server in a VM to handle some storage and backup along with running software that I don't have Linux versions for.

Like I said, none of this is for anything important or production.