I'd like to know what happened to her, but as far as her not making an appearance again goes, it's more realistic. Obviously the game is about a world with walking corpses, so it's not that realistic as is, but the Walking Dead has had a problem with improbable reunions in the past (Kenny, Morgan, Morales, Lilly, soon to be Dwight in regard to his casting in Fear).
Seeing Christa's storyline end would be cool and all, but it would be so unrealistic if she met Clementine in season 4. Kenny's reunion was unexpected but not totally impossible and Lilly as the leader of such a group was possible as well. But with the gunshot that shut off Christa's shouting while Clem was running away is for me the indication that Christa isn't alive and will never be heard of again