Oomis, thankfully thought, she over that. It is not a Yom Tzara just like the seventh day of Pesach is not a Yom Tzara although the Bnei Yisroel felt in danger. Besides, this is not the day that she fasted, but it is the day that the Jews were victorious. And the Gemara says that in some instances, the Megilla can be read on the 13th without a Remez in the Pasuk, based on the fact that this was the day of the victory.
One thing that makes me wonder is the Slichos that is said on Taanis Esther.