Could be the age thing, but I think it's more likely that the smirk's abut Tai having A girlfriend - after having been known around campus for her polyamory, the notion of her settling down may have inspired a "yeah, right!" reaction.
This is similar to my take as well, though the way I see it is actually a little bit meaner, and at Tai's expense: not only has she been known around campus, it's probably also a not-well-kept-secret that she was looking for something a little more attached and not finding it.
From that perspective, I'd find it reasonable to believe Allie's smirk is meant to be a knowing one with Dora, along the lines of, "Oh, you're the new one leading her on." Or, perhaps more charitably, "You're the one she's pining for this month?"
It could just be an insecurity issue, but...well, I suppose it's just arbitrary judgment on my part, but I think if that were the case Dora wouldn't have waited for Allie to leave to talk about it. She'd have blown up on the spot, in her face. Just guessing from past experiences; I suppose her counseling could've improved her impulse control in that regard.