Dinah had already had some wine while getting ready at Zatanna's. As a result, she was in a good mood and possibly already a little tipsy. She giggled when she spotted Oliver. Not that he was funny so much as she was curious to see Zee's reaction to this baby-faced Oliver. She had already mentioned that the Oliver here was from a different reality, but seeing him without his beard was definitely an interesting experience.
"Ollie, this is Zatanna," she introduced him. "Though..." she frowned. "Did you know Zatanna back where you came from too? This alternate reality thing kind of gives me a headache," she said to no one in particular.
It did, after all. She was just normally a little bit better at hiding that fact.