Yes, the straps of the brassiere form loops. A "loop" is a particular circular shape, while a strap is a piece of cloth used to attach or hold down something. They are not synonyms.
As in any language, English humor is often based on how certain words sound in context. "Loop" is a funny-sounding word, plus, since George's father is dismissively describing the shape of the bra rather than the function, it adds to the overall absurdity of the situation.
"Loop" is a singular noun. You can have a single loop, or multiple loops. It is also a verb, in that something can loop around something else, like a string when tying a knot.
Many other things form loops: bag handles, neckties, some earrings, certain types of pasta or cereal, etc. You can say that the orbit of the Moon forms loops around the Earth, and the Earth forms loops around the Sun. A happy dog can run loops around its owner. Airplanes can do an acrobatic maneuver called a loop-de-loop. And so on.
(Edit) in response to Lambie's objection: The scene isn't funny because "men are clueless". Seinfeld's humor was rarely that straightforward -- in the episodes where they did talk about how men know little about women, that was the premise of the running joke, not the punchline.
My interpretation is that George's father uses "loops" as a kind of dismissive oversimplification, because his ego was hurt from being wrong about what bras are made of. The overall scene is funny because of the absurdity of the family getting so argumentative over bras.
Note that Larry David, who co-wrote and co-produced Seinfeld, was the inspiration for the character of George Costanza. Judging from the kind of humor in his other shows, it's entirely likely that scenes like this happened frequently in his family.
