Mule allows the player to carry more, a very straightforward utility mutation.
The basement of Sharpton Interior in Gangland. From the north entrance to the basement, go south until you can go west, then north, and you'll be at the mutation spot. There are two locked wooden doors in the way, which you can break down with a bit of effort. The only mobs around are rats and chobos.
Adds 30% to your carrying capacity or 30 kg, whichever is higher. This negates the weight penalty for Lithodermis. The extra capacity is a direct, passive buff, so you don't need to store things in any pouches. Mule allows lower brawn characters to carry as much as their buff friends, whether for combat gear, loot, or scavenging. Even higher brawn characters get value; they can carry metric fucktons of stuff and do unconventional things, such as actually carrying around those 75kg incinerators.
If your carried weight sinks too low, you'll take minor stress. This usually only comes up when you've just died; just don't idle with an empty inventory. If you have Lithodermis, it's impossible to have this problem. The mutation can also be a waste of a precious slot for high-brawn characters who don't need the extra capacity.
Mule does not affect your appearance.