In English, apart from imperatives and some informal conversational patterns (see below), we always need a subject, even if it is the formally meaningless "it" or "there" (as in "It's raining", or "There is a book on the table"). In your example, "it" isn't meaningless, it refers to the skirt, and is required.
The informal conversational patterns I referred to are harder to characterise. In answering a question, we can sometimes omit the subject:
Why is he here?
Wants to ask you something.
but it is more common to omit the subject and auxiliary:
Sometimes in a sort of spoken narrative, the subject can be omitted when it's obvious, but this is nearly always a first or second person subject:
Went down to the shops yesterday: bought a sofa.
But here too, we often drop an auxiliary as well as the subject:
Been away, then?
Yes, visiting the Bahamas.
And I have had difficulty inventing a plausible third-person example, unless it follows a sentence where the subject is expressed:
She left home last Thursday. Went to London, apparently.
And I can't think of any plausible example of omitting a subject before a part of "be", except maybe in an argument:
You weren't there! Was!
In summary, I can't think of any context in which "Is her skirt" would stand as a sentence (not a question!) on its own.