It's a very common and informal way to ask someone how they are.
However, depending on the situation and formality, other ways to say "how are you" include:
* Aapay kyaa chho? (ah-pay kya chho) - more formal than "kasa chho"
* Aapay theek chho? (ah-pay theek chho) - literally "are you okay?"
* Aapan kyaa karee chho? (ah-pan kya karee chho) - literally "what are you doing?"
* Aapay kya aabhal chho? (ah-pay kya aabhal chho) - literally "what are you feeling?"
Remember that Kashmiri language is rich and diverse, and there are many ways to express a simple phrase like "how are you."