You are not unemployed when doing a PhD, nor are you goofing off. Not sure if your comment was supposed to be sarcastic somehow, but you are very misinformed.

I would say you are underemployed.

You are underpaid, but I would not say underemployed. The benefit society gains from a PhD is high, mainly because of the training students get from doing a PhD. But there is an over-supply of students who want to do PhDs as opposed to the budget for science, so the pay is very low.

I think lots of people get the idea of PhD is worthless learning from PhD graduate from some mediocre varsities. Getting PhD from any elite school (like MIT in this case) or elite research group is absolutely priceless.