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 (and being a percentage means it scales with brawn). It's worth noting that this more than negates the weight penalty of Lithodermis. There are no unwieldy commands to worry about, you just get extra carrying capacity passively (so no, you don't need to store things in any pouches). Allows lower brawn combat characters (blades, guns, etc.) to carry as much as their fist or club wielding counterparts, and for brainy characters to be able to scavenge more.
If your carried weight sinks too low, you'll take minor stress. This is only an issue if you die or go somewhere naked to keep from losing items, just be sure not to idle when you're not carrying anything. If you have Lithodermis, it's not usually possible to ever carry so little that you take the stress hit. If you have high brawn, the mutation's rather redundant, and taking up a mutation slot can hurt if you find yourself not needing the extra carrying power down the line.
Mule does not affect your appearance.