If you have a poem that deals with war, death is probably going to make an appearance. The theme of death is definitely at work in "The Death of the Ball Turret Gunner," as the title would clealry indicate. Jarrell wants us to consider life and death (mostly death) in this poem and he uses some contrasting imagery (birth/death) to encourage us to contemplate these big ideas. And just to make sure we consider mortality, Jarrell chose to make the poem's speaker a dead soldier. Not too subtle. If you were looking for light reading, this ain't it. (Okay, you're right. It is short and therefore light in the sense of not long, but this one packs a heavy thematic punch.)
No big whoop. Death, whether in war or in old age, is just a part of life. The ball turret gunner's death is therefore no more tragic or unfair than another.
Actually, this poem is a big whoop. Confronting the idea of death, whether through art or religion, enables us to live fuller, more meaningful lives.