Is 笨蛋 a derogatory word?
Think of the phrase "You fool!" or "you idiot" -  depend on the relationship between the two people, the reason behind saying that, and the tone of the speech, it can be a playful, affectionate flirt,  or a contemptuous comment.  

笨蛋 is such a word.

Of course it is a degrading term. But 笨蛋 is not a word of hate in general. Only best friends would call each other fool or idiot casually.