Casey at the Bat is one of my all-time favorite poems. I always thought it was just about baseball.
But is it? Is there a metaphor or an allegory hidden somewhere in the last inning of a baseball game?
It's not an allegory, but it is a common feature of life. It's not "just" about baseball: it's about disappointment, about expectation, about confidence, about the trope of overcoming adversity. Sometimes, you just lose. It happens.
There's no need to apply an allegorical reading where the baseball represents, I dunno, the Free Coinage of Silver and Casey represents William Jennings Bryan. (Example borrowed from Cecil Adams.) The poem is appreciated by people who aren't baseball fans because they know the bittersweet feeling it evokes: the heartbreak of loss combined with the silliness of the poem's style. It reminds you that there's another game tomorrow, and life goes on.
True allegories are actually quite rare. The trope rarely works well, because no two situations are ever truly identical, and trying to force them to be just makes people uncomfortable. But great works capitalize on common human experiences. As they say about history, it never repeats itself, but it rhymes.
No, there is no complication regarding the theme of Casey at The Bat. The poem as generally known is purely about baseball and nothing else. However, it is unclear whether this is a portrayal of a specific game and player or not, but we do not have enough information to determine if it is and if it is, who it is. Additionally, in a 1979 article, sportswriter Leonard Koppett claims that there were 19 lines originally written, but omitted from the final version, which gave the poem and new meaning about gambling, but there are no corroborating sources to this claim.