she is an assassin. she should be feast or famine. she should have good base damages but bad scalings to make her have to snowball into the midgame and less relevant late game. Right now she has the best of both worlds and it is making any player who gets further than the basics on katarina able to get very high winrates due to her having no real weak point in the game.
Katarina isnt an ungodly monster that is unbeatable early but she does have some pretty good in lane harass and she is pretty safe while doing it. Once she gets Hextec revolver (not gunblade) she gets a HUGE powerspike (This is like maybe 8 min in so its still early game) With this item alone she actually becomes very powerful in the lane phase setting. and she only gets more powerful from there. Midgame is where assassins shine. All of them. Including Kat. Late game she is still amazing due to her damage being so hyper mobile AND AOE. After she has done the dagger combo in a teamfight which will have killed 2 squishies, she pops ult to finish the job.
Lets compare this to zed. Zed is semi mobile due to all his mobility is stuck on one of his main combo pieces that DOES NOT RESET. He is pretty good in lane and during the midgame he is a monster. He is able to oneshot most champions as long as he didnt fall behind. Late game though he loses some of his presence and because his ult makes him a sitting duck for CC he becomes possible to deal with.
Katarina doesnt have this kind of counterplay. The daggers are not nearly as easy to dodge as everyone is making it seem, and the thing is dodging them doesnt even reward you. it just means you didnt get one shot because she still has enough follow up damage to kill you since instead of fighting her you have just been running away. She has been attacking you this whole time and just waiting for cooldowns. you have to 100-0 her in about 2 seconds after dodging the initial dagger combo (and you havent hurt her during this time cause you have been running to dodge the daggers) or you have to repeat the fucking process. If her main abilities had significantly longer cooldowns to offset the power of the daggers that would be 1 thing. But they are standard length cooldowns.
Kat doesnt have windows. Zed for instance has a long W cooldown that you can play into. Kat has reletively low cooldowns for an assassin. If they upped all the cooldowns on her that would be one thing. give a larger window to fight her. but as it currently stands the window is so incredibly short that only super meta mages like ahri can actually do anything about her.