Hello.
Installing the plugin directly in Picard places the plugin in ZIP format in the directory:
C:\Users\***\AppData\Local\MusicBrainz\Picard\plugins\
Should plugins installed from my disk also be in ZIP format?
Firstly there is not a single zip format. The plugins are generated in a specific format from the Github repo and Python knows how to read that format.
There may be other zip formats which are also supported, but equally there are likely several zip formats NOT supported, but I am unable to enumerate either group.
When I mess with my plugin I just zip with standard default .zip format as found in the 7zip app.
Simple answer - if it works, then you have the right one.
ZIPping the plugin is optional. For locally installed plugins I would usually recommend just keeping the files locally. It makes it easier to edit the plugin.
There are two types of plugins: those that only consist of a single .py file with the name of the plugin (yourplugin.py) and those where there is a folder with a name of the plugin containing a file called __init__.py. The latter allows the plugin to consist of more than one file (but it can also be just a single __init__.py)
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As I said, if it works it works. .zip is an old enough standard that means built in Windows tools have handled it for a while now.
7Zip’s app is a very multi-purpose tool.
I have the official paid for DOS floppies for PKZIP around somewhere… 
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DOS rules. 