That's just Tataru though, she's always had rather goofy facial expressions even back in 1.0 - I'd put it down to just the engine not rendering her smile right, creating a slightly 'uncanny valley' effect.

As for your theories about her being a mole, that doesn't hold weight I'm afraid. Tataru wasn't at the banquet for the same reason the other minor Scions weren't present. The banquet was ostensibly for those who directly participated in the defense of Ishgard against Vishap, hence only Minfilia, the Archons and of course the Warrior of Light attended. Tataru is just a receptionist - therefore she didn't warrant a place on the invite list.

Besides, Yugiri confirmed Tataru's whereabouts during the banquet - she was actually in Limsa Lominsa's market with F'lhaminn when the Crystal Braves went rogue and F'lhaminn distracted them so she could escape, and from there Yugiri escorted her to Vesper Bay. There was no ulterior motives in Tataru's behaviour, she was innocently shopping when all hell broke loose.