Other mistake: When Joe falls on the bungie, we see him hit his head in an entirely vertical position with his head at the top. This could not have been achieved considering the position he was in in the shot before.
Other mistake: When the old man gets his foot caught in the railroad switch, the switch closes on his foot, trapping him. As the camera pans out we see that this was not an electric switch but a hand throw switch, meaning that someone had to be standing at the switch manually operating it to close on the old guy's foot.
Answer: It's not so much a joke with a punchline, but more like a funny phrase. He was affiliated with the mob, and as such, has most likely been in violent situations. He asks "does your mother sew" and then punches his refelection in the fire extinguisher case (someone's face if a fight was really happening). The "get her to sew that" comment implies that the person he hit would have needed stitches. (thus the reference to sewing.)
Jazetopher
He was referring to fixing the hole left in the clothes after being shot.