A generic UserTemplates topic name of the CommentPlugin
could belong to any extension or even the core.To avoid confusion I suggest to rename/move the template topics as follows:
Reason: The Main web is not touched on TWiki upgrade, hence it makes sense to put the user defined template to the Main web. This is consistent (& orthogonal in literary sense) with TWiki.TWikiPreferences Main.TWikiPreferences.
I'm not convinced that this is a good idea. All plugins write to the TWiki web; recoding them so that they assume the name of a second web (Main) doesn't sound like a good idea. These topic names are highly unlikely to conflict with other topics, and UpgradeTWiki
is smart enough to steer around them.
This is your plugin and you can do whatever you want. I just thought that the name TWiki.CommentsTmpl is not very descriptive, and TWiki.CommentPluginTemplates indicates by topic name where this topic belongs to. The name "CommentsTmpl" sounds like part of the documentation of the core TWiki.