Daily misting is of little value in raising the humidity. The humidity is increased only as long as the water droplets remain visible on the leaves. That may be as little as 10 minutes out of a 24 hour day. So, yes, a closed terrarium is a much better way to maintain high humidity for your Fittonia. That said, if you notice water droplets forming on the inside of the glass cover, then you will need to open the top a bit to decrease the humidity a bit.
