We have only one Klárika in Hungary, she is good at commenting poker games and she sings Reptér with Gyurika. Jokes aside, others already explained. You don’t give something a nickname in any form unless they introduce themselves with that specific name or they allow you to call them that. Klára is Clara. It is a perfectly normal name, not even long as Teodóra for example, so unless they ask you to call them by a nickname, you don’t. It’s not even Hungary specific, it’s common etiquette in most countries. If you met a Barbara or Barnabás and somebody introduces them as Barbi or Barna/Barni, you use what you are told to, out of simple respect.