A salty person is someone that is angry or ill tempered.
If you become angry or ill tempered easily then you would be called a salty person.
Being called a salty person is not a bad thing but it just means that your personality is bitter or angry, irritated and hard to take.
If someone does something to make you angry easily and you became angry or irritated really easily or cannot take criticism too easily then you would also be a salty person.
I've been called a salty person before and I do sometimes get angry easily and so being called a salty person to me is not bad because it's true.