Positive. Watch Julia Gillard’s 2012 ‘misogyny’ speech to the Australian Parliament. That’s what formidable looks like.
When somebody wants to accuse a woman of being strong, they don’t tend to sugar the pill. If the person using the ‘formidable’ description wanted to say ‘X is a harridan’ or ‘X is a virago’, then that’s usually what they would say. If they understood what those words meant, of course.