In He-Man is skelator an undead skeleton or a lich?

Probably more likely a lich.
Guy is as jacked as prime-Arnold.

A body-builder (in Lycra) with a a skull mask.

A lich. Or a guy in a mask. Or a guy whose face was burned off by acid. He’s whatever the person writing him at the time wanted. They never really settled on any kind of fixed origin and Mattel didn’t give a shit as long as the toys sold.

I think he was a lich. His face was disfigured by what appeared to be acid. Then he sold his soul which left his face as a skeleton’s frame. So he was an undead that was transformed by magic. Here is the introduction to Skeletor