Sure, you can say that in that way, but of course, it needs to be in the right context. Not so much to mean, are you an idiot, but rather to mean, are you sick in the head. I would imagine a scenario like the one person talks about putting something unpleasant in another person's drink, and the third person then says, 'ben je ziek?', like English would say, 'are you sick?' But you'd more likely ask, 'ben je gek?/are you crazy?'.