John Updike's poem “Ex-Basketball Player” is about a man named Flick Webb. Flick was a high school basketball star who now works at a gas station. Updike's purpose is to show how fleeting fame can be, especially for someone as young as a high school star.
Flick is obviously living in the past, as evidenced by this line:
Once in a while,
As a gag, he dribbles an inner tube,
He also imagines he still has an audience watching his every move, just like he did when on the court in high school.
Flick seldom says a word to Mae, just nods
Beyond her face toward bright applauding tiers
Of Necco Wafers, Nibs, and Juju Beads
Your question specifically addresses the first line of the second stanza,
Flick stands tall among the idiot pumps—
At first, the reader might not realize what Updike is getting at here, but the rest of the stanza goes on to describe what the pumps look like:
Five on a side, the old bubble-head style,
Their rubber elbows hanging loose and low.
One’s nostrils are two S’s, and his eyes
An E and O. And one is squat, without
A head at all—more of a football type.
“Five on a side” — just like a basketball team. The line “rubber elbows hanging loose and low” personifies the pumps in terms of what a basketball player might look like — long and gangly.
Now that we've determined Updike wants to associate the pumps with human basketball players, we may well conclude that, by referring to the pumps as “idiot,” Updike is suggesting Flick may also be an idiot in some way. Exactly what way is up to the reader to determine, but the poem suggests that it is wise to think ahead, to plan for a life without basketball. Flick feels he peaked early — what is left for him now?
I would imagine Updike is thinking in broader terms than just Flick; he's probably thinking of the high school athlete in general, the kind that gives no thought to his or her future beyond the brief years of heroism high school can provide.