It's important to note that "Catfish in the Bathtub" is not an essay but an excerpt from the chapter called "Shaman" in Hong Kingston's autobiographical book The Woman Warrior. As such, it is highly unlikely that the excerpted section that runs from "My mother cooked for us ..." to "I would live on plastic" will have an independent theme of it's own as it is a thematic part of the whole "Shaman" chapter, which itself is a thematic part of the whole book, with its overall theme.
The line, "I would live on plastic" actually extends to "My mother could contend against hairy beasts ... because she could eat them." Two things come to the fore from this in terms of overall theme. The first is that "plastic" introduces a recurring thematic motif and that "eat them" emphasizes the theme that it might be said this excerpt dramatizes.
When looked at in isolation, this excerpt (not essay) seems to relate the impersonal to the personal by having the impersonal (a fish) swimming in something personal (your bathtub), noting that "bathtub" also represents a recurring motif in the book.
The connection between the personal and impersonal becomes stronger when you consider that the impersonal fish becomes very personal food for sustenance of life. A possible theme emerges from this connection that might be stated this way: The impersonal becomes the core of the personal through the incorporation of the impersonal into the life substance of the personal, for example, by impersonal elements of nature becoming food for life.
Another possible theme is that under extreme circumstances, even the impersonal extremes of life, like plastic, are preferable to grotesque personal associates, like the association of catfish in the bathtub.
... I have seen revulsion on the faces of visitors who've caught us at meals.
"Have you eaten yet?" the Chinese greet one another.
"Yes, I have," they answer whether they have or not. "And you?"
I would live on plastic.