I think that at some point, you have an obligation to ensure this doesn't happen, and not just 80% of the time. If your child hits another child or tries to hit another child every time you're in a group play/public setting situation, then it's unfair to put other kids in the line of fire, so to speak.
Now, I have no idea how to accomplish this, as it also seems mean to just isolate her until she's past the "smack all the girls" stage. Once the smacking has already happened, though, I think you are fine to apologize (as this obviously was not intentional on your part) and move on from there.
If I kept seeing you at the park and I knew your daughter was the hitting daughter, I would be inclined to keep my kids far away.
...and this is why having kids seems really, really hard. I hope this passes swiftly, OP!