I love this name and the meaning. It's quite beautiful. I love it so much. But I hate the name Vanessa. I know that if I name my daughter this when she says "I'm Nessa" everyone will be all "Oh Like Vanessa?", I do though love it for a character.
I like the name Nessa and the meaning of miracle is my favourite. I use the name Nessa-Rose for a character in a story and I've only just learned that there's a famous character in Wicked named Nessarose.
If I was to put any associations of the character from Wicked aside, I would think this is a very good name. And the meaning of miracle is fantastic (at least I think so). It suggests the color pink to me.
The Hebrew word for "miracle" is "ness", which as a name is pretty rare but can be used for both genders (though I've only ever met one Ness, and he was male.) "Nessa" would be a strictly feminine variant.
In actuality Nessa reminds me (somewhat) of the "name" of the Wicked Witch of the East according to Gregory Maguire's 'Wicked: The Life and Times of the Wicked Witch of the West', which is Nessarose. Nessarose was born unable to walk due to her mother's eating milkweed during pregnancy to avoid another green-skinned child like Elphaba (Witch of the West), and due to the somewhat-miracle of Elphaba enchanting the silver/ruby shoes their father had given Nessarose, Nessarose could now walk. That's what Nessa, meaning "miracle", reminds me of.