The amount of uncovered skin on Kai'sa's outfit in terms of how low the neck goes is pretty common for female formal dress apparel (yes she is not wearing a formal dress, but thats not the point). Her personality is very much non-sexual and she does nothing to flaunt her physical body (unlike champions such as Ahri and Eve)
For some reason the word "sexualized" is being used as a synonym for "attractive", but with a negative connotation. i don't get why the small minority of people that get offended at any character thats made to look attractive have so much weight.
It pretty much seems like Riot is not allowed to make an attractive female champion anymore unless their lore is "im a creature that transformed into an attractive female human to lure people" (which is the lore for both Ahri and Evelyn which is the only examples they have ever given for champions that have been "ok" to be attractive)
So looks like females are not allowed to be sexually attractive unless its to reach a specific goal (to bait people)