I have strict parents as well, but calling you that way is extremely unpolite! Just prove him he is actually wrong!
Don't be concerned! i mean.. after all it's just his opinion, right? it's not a fact or anything. If you don't feel stupid then there is no problem.
I think that it may be your dad's way to say, "work harder." I'm not sure, but some people aren't born gentle. They're...just...tougher, I guess. Strict parents are a pain, but they're doing this all for your good. Sometimes you wish that they wouldn't mind you at all, but it's actually to encourage. Or so I hope. *o*
It just sounds like they are trying to push you a little to hard. Being mean is not a very good way to push a child. It might be good to talk to your parents and tell them how things like that make you feel.