We have an idea of a lot of order members' skill set despite the PoV. Most of them have a word of mouth moment and a we can deduce what happened moment. And, as the other person said, there are plenty of opportunities for Molly to do things. She's never chosen. Not to watch privet drive, not to escort Harry to Grimmauld Place, not to guard Hogwarts. Nothing.

Furthermore, there is no measurement of 'good enough.' They were vigilantes and Dumbledore can't be choosy with willing and active bodies in a war. It would be like saying Colin was good enough for the DA.

Full stop there is just no build-up for Molly Weasley being a witch talented enough to throw curses with Bellatrix Lestrange and make the floor crack. And, no, the power of love doesn't cut it when we saw Bellatrix leave Alastor Moody crawling on the floor.