Something semantic would seem better, yes.
I disagree. The feature isn't specifically about administrative
categories, it's about hidden categories, so HIDDENCAT makes sense.
You could have an alias, I suppose.
Are there other meanings
for which you'd want to hide the category?
Who knows? People use Mediawiki for all kinds of things.
And should stub categories
be hidden by default?
Does the phrase "stub category" have any meaning to the software?
Aren't they just like any other category? In which case, whether or
not they're hidden is a matter for individual wikis.