Our Last Braves Game

[Kynaston Levitt]

I don’t know whether there’s a contextual joke in the (Atlanta) Braves vs South Decatur, and whether the latter hosts a rival American football or baseball club.
Can’t understand the first two lines either - what ruin, and what crack? Why mention the normally expected two way loss?

Why would you need to know “what ruin?” and “what crack?” to understand this? 
I’m sure you do understand it, but you just don’t know what specific ruin or what specific crack… because I haven’t put those bits in. I leave it up to you, dear reader, to imagine the ruin and imagine the crack and imagine what those words might mean in relation to the theme of the poem—they may even mean more than one thing at a time!
As for the second, in terms of relationships, I’m not sure it is an expected two-way loss. Often, someone gets dumped, and they are the loser. In fact, “it’s their loss” is a common refrain of the friend or relative of the dumped. Because to lose something implies you cared to have it. You seldom really lose what you throw away. So, I thought I would balance the equation like that to show the insecurity of the author. Also, it implies she's not dead (this isn't a dead girlfriend poem)... and, lastly, I think it’s funny humorous word play.
And no, there is no contextual joke in the title. In fact, it's about as straight forward a poem as a poem can be, so I'm a little surprised you're struggling with it.