The charwoman was actually the replacement for the in-house maid which had to be let go. The story describes her as
This old widow, whose strong bony frame had enabled her to survive the worst a long life could offer, by no means recoiled from Gregor.
Basically what this states is that she is a typical and provincial character which lightens the story since she maintains a static behavior and does not deter from it. She is the typical tough old lady who has lived through too much to bother with something weird. She sees the real Gregor, and not what he has become. She actually teases him and, unfortunately, was also the one who disposed of his body when he was found dead. For her to be able to perform these tasks without flinching is clear indication that she was tough as tough can get, and that this toughness is what gives the humor to the story.