Drop rates are a hard thing to balance. But when the only really relevant "drop" from fighting bosses is in one category, girls, it becomes simply frustrating to get bad luck. If there were other things that dropped from fighting bosses that were at least worth taking note of, that would be a very pleasant surprise and at least make the grind less frustrating.
In short, I believe the low drop rate on girls to only be a problem because after grinding a boss for 300 misses, you really have ZERO to show for it. Nothing at all. (Aside from low level items that you don't really need.)
I don't think the drop rate of girls needs to be changed, but here are some of the better ideas in my opinion. (Please note that the second idea I came up with independently from Raziel1806. It was coincidence that we came up with the same idea, but I am willing to give credit to the first person who posted an idea.)
1. Cosmetic items as rare drops, almost as rare as girls but maybe twice the chance. Because they are also very rare, it would still feel like at least you got SOMETHING. These items could (maybe) be used to modify your avatar picture, which would be really nice because my guy looks like a total idiot. Also, since it is almost as rare as a girl, you would feel like you just narrowly missed the girl. You could almost taste her. Maybe make them... a piece of the girl's clothes? (It came loose during the battle perhaps, and she forgot it?) But probably not both wearable by you and a piece of her clothing. Unless players want it that way...
2. The drop rate of the girls go up a tiny, miniscule amount each time you beat the boss. Thanks Raziel1806, great idea! - He suggested 0.01% but I believe this is too great an increase. Even a much tinier 0.001% would work. It makes an extremely little difference, but it's about the psychology of it. At least you know you are getting something. And after 300 misses, you would have an increased 0.3% chance per boss win to get a girl. This is actually relevant at this point. Maybe don't show the base chance to get girls, since that's a secret, but show the increased chance that you have obtained through hard sweat and effort. (Also, this number would be different for each boss and could reset on getting a girl.)
3. Rare item drops that let you do unimportant things, like "automatically collect gold from girls for next 8 hours" consumable, for example. Money really doesn't matter, but it's nice to have it auto collect over night. Or maybe an item that changes your character's main stat? It doesn't really matter to me, but being a shagger wouldn't be my first pick. If you make these drops as rare as girls, they won't really affect the game that much. But at least you have a chance for something besides a girl to drop, so there are more chances for players to get something and thus not feel frustrated as much. "Well, at least I got this rare item."
Those are my two cents for now.