Factual error: At the end when Joe, Brandy, Clem and Charlene, the alligator woman are in Joe's 1967 Plymouth GTX, Robby claims his car is faster. Charlene chides back about his Trans Am, saying 'you think you can match that little old slant six against his 426 Hemi?" Pontiac Trans-Am's did not come with slant sixes, a Dodge motor. His was obviously a V-8, and would have most likely have been a 400, but we get the 'yours is really small' reference.
Factual error: When Joe is at the dealership talking about the Rambler, the dealer says they came with a 289 automatic with 3 on the tree. It is impossible to have both an automatic transmission and a "3 on the tree" which is a manual transmission, at the same time.
Factual error: In the scene where Joe Dirt is talking to the sales rep at the dealership about who ordered the Rambler wagon, the sales rep mentions it came with a 258 cubic inch engine. No Rambler ever came with the 258 six. American Motors (makers of the Rambler line) came out with the 258 in 1971, 2 years after the last car with the Rambler name (the Rambler American), was sold in 1969. Although I was impressed that they bothered to mention an actual American Motors engine, it's still not correct.
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.