I've been playing since September and I still don't have a single girl to drop, so I'm wondering if it's because I skip my battles.
Regarding the event, I think it's a definite improvement over the Halloween event. Six new girls is above and beyond what I expected, and having one of them available without a paywall or dealing with RNG is fantastic. Having two RNG girls is nice too, though with my recent luck (Gruntt has only dropped one girl for me since the Invaded Kingdom was released, and that happen until at least two missions of Juy Sea were out) I'm concerned that I won't get either of them... The kobans girls all look enticing, but I probably won't bother. All in all, I'd say it's very good as a second event!