a mule to me is considered any character you use to do something you need to do for another character

the most common would be a crafting mule to make armor for another character

another which people dont often think of is a character that goes out to make money and gather resources for a character that enjoys crafting but has no fighting skills, some people do play characters and have fun crafting things rather than having to fight.

a mule is just a character that does something you dont really want to have to do but do it because its easier than looking for someone else to do it or having to pay for it to be done.