I'm mostly familiar with tar and feathering episodes from about mid 18th century to mid 19th century in North America. There were other times and places where it was done, and this may not apply to all of them.

In general tar and feathering was not intended to be lethal. In fact I'm not familiar with a single incident that was lethal. Even particularly vicious attacks like John Malcom's in Boston 1774 wasn't lethal, and he suffered a pretty severe beating and exposure to go with the tar and feathering. He even kept scraps of his own skin that tore off when he tried to remove the tar to show to people, and demonstrate the barbarity of the colonials. But he survived another 14 years after his attack.

Instead tar and feathering was meant to be painful and embarrassing. It was a public humiliation, and victims frequently spent some time on display, either in a pillory or even driven around in a wagon.

The tar used was usually pine tar, the kind used on ships. Pine tar was used on ships because it was used to treat ropes and had to remain soft and pliable at room temperature. So unlike petroleum based tars which have to be heated up to 300° or more before they'll melt, pine tar melts somewhere between 100° and 140°, or somewhere around the temperature of a cup of coffee. Some descriptions of tarring do include mention of burns, but many don't. And when burns are mentioned, they generally seem to be second degree, blistering, not charring. So definitely painful, but not likely fatal.