Fantine was a bit taken aback by Maria's question.  Recruiting (sort of) Chloe was something Maria probably could be proud of, but Fantine didn't see that keeping Eponine's loyalty was something Maria should gloat about.  If anything, Maria should be showing relief at having kept Eponine's loyalty, and maybe even a little shaken that she had risked losing it in the first place, but the way Maria asked this question implied that she felt neither.

Of course, Fantine chose to indulge Maria's question, "If I were to guess, I think you're happy because you managed to make that other magical girl join you." Fantine chose to leave Eponine out of her response.  She hadn't seen her sister so emotional for a long time, and didn't like how it seemed that Maria was taking such a powerful and shaking moment for Eponine and chalking it up as another 'achievement'.  Though perhaps Fantine was misreading Maria's demeanor; Maria's response may reveal something else.