The headline is not a joke. Obama’s tax plan reduces the incentives for high income earners to work. Therefore, many won’t have incentive to work harder and will spend time in other pursuits. Here’s Mankiw:

The bottom line: If you are one of those people out there trying to induce me to do some work for you, there is a good chance I will turn you down. And the likelihood will go up after President Obama puts his tax plan in place. I expect to spend more time playing with my kids. They will be poorer when they grow up, but perhaps they will have a few more happy memories.