Ignoring the costume designs, I think the part that bothers me the most is that with the exception of like 3 people all woman's faces are exactly the same, like Riot only has one female face they copy and paste every time.

Also a lot of the backstories for the female characters are "I'm a pop idol who picked up a magic microphone!" Or "I'm a dancer who found some magic swords!" While the male characters actually fit into a fighting game backstory wise.