What's the theme, based on the topic of abuse, of the poem "A Smile to Remember" by Charles Bukowski?

The title of "A Smile to Remember" is ironic. Irony occurs when words or events in a piece of literature means the opposite of what a reader expects. In this case, the poem's title suggests a happy poem about pleasant memories. The first few lines also lead us to believe this will be a poem about happy memories:

we had goldfish and they circled around and aroundin the bowl on the table near the heavy drapescovering the picture window andmy mother, always smiling, wanting us allto be happy, told me, 'be happy Henry!'and she was right: it's better to be happy if you can

The theme of the poem, however, is painful. The poem describes how the narrator's mother coped with the physical abuse she and her son endured at the hands of his father—by counseling her son to "smile" and to "be happy." She asks her son why he won't smile. She keeps smiling, although her husband beats her and her son two or three times a week. Her son says her smile is "the saddest smile I ever saw." We realize that what the mother says about being happy is simply a front meant to paper over abuse and misery.
The narrator likens his mother and himself to the pet goldfish they own. Like goldfish, they are trapped, with nowhere to hide. The mother's way of coping, which is denial of how miserable she and her son are, is not adequate. The son, at least, acknowledges he is unhappy by refusing to smile.
At the end of the poem, the narrator tells of their goldfish dying. His father throws the dead fish on the floor and feeds them to the cat. This shows the father's lack of empathy. Although this is a sad and callous event, the mother continues to smile.
The speaker acknowledges early in the poem that his father suffered from psychological problems and anger issues he couldn't understand:

raging inside his 6-foot-two frame because he couldn'tunderstand what was attacking him from within.

Nevertheless, the mother, by not being willing to confront what was going on, condemned her son to an unhappy childhood. We are left feeling sorrow for the son, who has been abused by the father and left unprotected by the mother, and frustrated with the mother, who was unable to face what was happening in a healthy and assertive way.