Do the narrator and Sheila Mant make a good couple in "The Bass, the River, and Sheila Mant"?
Definitely not. We can see that the attraction the narrator has for Sheila Mant lies primarily in the spell she casts over him, the way that she is associated with beauty, parties, and popularity. Consider how the narrator views her at the end of the story, just before he cuts the line, in the moonlight:
Not just Sheila, but the aura she carried about her of parties and casual touchings and grace.
It is this that makes the narrator attracted to her. As the story progresses we see Sheila for who she really is: a egotistical, narcissistic and self-obsessed individual. Note her reaction when the narrator tells her there are bats. Also consider the way that when she talks she only refers to herself. The following is a classic example of her selfishness:
"I have to be careful with my complexion. I tan, but in segments. I can't figure out if it's even worth it. I wouldn't even do it probably. I saw Jackie Kennedy in Boston, and she wasn't tan at all."
This shows how she is driven by fashion and popularity, and thus we can understand why the narrator symbolically cuts the line, letting the tugs on his heart that Sheila's attractions exert on him vanish and disappear.