her backstory?  
She is a mediocre pony, she live a normal life, taking jobs to live, most of the times are really simple jobs than anyone can do. But something always happen and she lose her job to search for other one.  
She have really bad luck, because when she put effort and try to achive something great, something else always happen and ruin everything.
 
she have no cutiemark because she have no talent… or at least not one she can exploit yet….