Kat's my main... easily countered if you know how to. Most people panic when they see they're against a Katarina so they tend to not react quickly enough. She has very low health, her cooldowns are a bit long early game, too. She's just very quick in combat.
Not every Kat is great, tho. You can't go in too early, or you will die because all your abilities will be on cd so unless she has flash up she won't be able to escape without help. But when you go in at just the right moment when everyone's used up their abilities &/or are low, Q-E-W-R & those resets will help you get more kills just in case your ult doesn't finish everybody off. It only hits the 3 nearest people.
Most people act like you click R & automatically teleport to the team fight & get a penta... way more work than that, even tho it appears to anyone who doesn't use her that all you have to do is R. Shut her down early with your jg & you'll have no problems.