I called a customer's dad and told him he was stupid
Why did you call up a customer's father like that?
I was in a bad mood.
You did wrong here because it's rude and immature to call up this person and tell him he is stupid. Even if you are in a bad mood you should not have done that. Plus, are you not afraid that the person calls back to your employer and explains what happened and gets you fired?
They will not do that, they don't know who I am.
You took a risk that should not have been taken. You should apologize to the person you called and not do that again. That is very rude.