I don't really see a huge issue with having only 25% of the roster as female characters. Part of this is why I think they have a lower ratio,($$$) part of this is why I don't see it as a problem.
More men buy games then woman. Yes, there is a ton of woman gamers, but not many as men, and I'd wouldn't be surprised if the gender gap was even greater for people who buy MK games. It's not just a video game, it's a fighter with extreme violence and gore, that's something that generally skews more towards men's tastes.
When MK was in it's early days, the men to woman ratio was dramatically more skewed towards men, particularly boys. It's not like today where there is a seriously large gaming fanbase who are female. You sell to your demographic.
Because of this, the majority of characters they've made have been men. Since the majority of the characters are men, naturally, the majority of good characters are men. They've had more tries. So in general, the most desired characters by fans are men.
Many of the woman characters they have made aren't good characters and even the good ones tend to be overlooked in favor of other male characters by the fanbase.
They've done a great job with the female characters they do have in MKX, adding a few new ones and making them super relevant. Cassie gets the chapter equivalent to Raidens.
Now, if they are going to make new characters, by all means, get some more women in there. But for returning fighters they just have a ton of great male characters and only so many great female characters. And in MKX, they added 8 new(non guest) characters, and 3.5 of them were female. That's a good ratio considering the points I listed.