The emote ‘Wow’ is one of several Battle Emotes that you select to use as a customisation item on your Ninjala character.
By going into the Closet and then the Battle Emotes menu, you can change your defeat emote, victory emote, and your avatar’s actions when a match is about to commence.
‘Wow’ Official Description
“An eager and rousing cheer. Let others know how amazing and impressive they are.”
Your ninja gets very excited with the Wow Battle Emote selected. This emote was probably designed to help your character celebrate entering into the Ninjala tournament.