Don't listen to people that "insult" you by saying a champ is OP, easy, etc. They are just mad that you killed them. Kat is not difficult because she has no skill shots. But she is difficult to play the more advanced player you are against (she is very susceptible to CC and you WILL be a target and they WILL save their CC for your ult). There is also a lot to learn about her, such as ward jumping, managing cooldowns, knowing how to team fight, not die, then mop up the team fight.
Don't worry about what other people say when you kill them...if you enjoy a champ and play them well, then keep doing it.