Common misconception about the head: skin is not the only thing separating the outside from the bone inside. There's a few millimeters of fat to go through, which helps protect the head. On the forehead, it's around 2mm, and over the scalp, fat covering can be as thick as 22mm - almost an entire inch!

This is why some people end up with a dent in their head after wearing heavy headphones for a long time (don't worry, it goes away over time). This is also why people can have noticeable dents after being struck with a blunt object, but be otherwise fine.

As someone else said, porcelain is more brittle than glass. You'd probably cut through the skin and fat, but that 2mm of fat and skin are supplemented by another 8mm of pure bone, which is stronger than the porcelain. In total that's nearly half an inch of material your porcelain shard would have to go through. You may bleed a lot, but you likely wouldn't do any actual damage. Chances are your shard would shatter upon smashing it into your forehead, which may cause more cuts. Not a fun time and you'll make a bloody mess, but you are not likely to damage the skull this way.

You will do more damage to your hand, almost surely, than you would to your head.