She is good, but she flies by the book. I never thought I'd see the 'Kirk Maneuver' flown mechanically. She's conservative in open battle. If there is a classic solution, she will employ it flawlessly. If she gets orders from a superior, she will implement them well. You can't fault her reaction time, or her sense of timing. She also just does not get disoriented, no matter how twisty the fight gets. She has that magic something called 'situation awareness'.
But if red force sets a trap based on her group following traditional doctrine, she will wade right into it. She worries too much about taking casualties, so she won't take a risk. If you set up a scenario where the only way to win is to break the rules, she is dead. Her nemesis is a simulator program called 'HoloKirk'. She isn't alone. There isn't a cadet in the place that does well against 'HoloKirk'. Joy has only managed to beat him once. She got mad early. Don't ever get her mad.