In the poem "Snaps," what are the ethical statements in relation to gender, family, and convention that the poem tries to make?

In "Snaps," Espaillat contrasts the conventional expectations of how a family wants a young woman to act with her subtle signals that she is rejecting their conventions. "Good" is the pivotal ethical word in this poem. The poem begins with the word "Good" and then goes on to describe what a conventional good "little girl" looks like to her family: "how neatly dressed you look." Here goodness is equated with outward neatness and conformity.
Similarly, her 17-year-old self is praised for conformity to gender norms and the outward appearance of goodness. She is a "nice girl, with ankles crossed...gold cross against the clean sweater." The lap is described as a "small bounded nest," a metaphor that indicates the girl is closed in to a small space with boundaries, the word "nest" an indication of home. She is expected to stay within a small, limited world defined by her home. On the outside, she displays the picture of cleanliness, conventional Christianity ("gold cross") and normalcy, demure with crossed ankles and hands folded in her lap. 
But at the end of the second stanza, the poem begins to introduce some uncertainty about this picture of perfection with the words "but perhaps," which pull us into the third stanza. Here we begin to see that "they knew less than they thought." Behind her "young, obedient pose" (the word pose indicating that she is putting on a performance), the camera captures another side of her: "a strain ... a certain look." The narrator uses the metaphor of opening a "plain old book" and being unexpectedly surprised by the words found there. Where, the narrator asks the young woman in the photo

did you get it, that sharp tilt of jaw
small thrust of hip and shoulder not in keeping
with Mama's touch that bent you to the law,
to sanctities of custom?

In small, subtle ways, the woman in the photo defies gender expectations and expectations from her family ("mama") and "custom" that she be docile and obedient.
The last stanza returns to the moral/ethical term "good," redefining it. The conventions (that she not talk back, that she not cross lines forbidden to women, that she be docile, that she not "risk anything"), all of which add up to "good," are questioned in the poem's last line, where the narrator says that the woman in the photo is "flashing me a sign ... that yes, we would." This woman says yes, she will take risks, she will dare to step out of line, dare to talk back.  
The language of enclosure and conventionality used throughout much of the poem--"good, neat, obedient, small bounded nest"--helps the reader feel that in breaking out, the woman is doing something morally better than merely being "good." She is finding herself, becoming free of others' expectations--and that perhaps is best thing of all.