You’re reading way too much into it - it’s just a colourful expression. Anyway, “double ungood” would not be a valid Newspeak construction. The base is “good”

You’re reading way too much into it - it’s just a colourful expression. Anyway, “double ungood” would not be a valid Newspeak construction. The base is “good” and “ungood” is the negation; intensification would be “plusungood” and further intensification “doubleplusungood”.