She’ll presumably be mourning the death of Viserion in Dragonstone, although it’s strange that she wouldn’t be present in the meeting at the Dragonpit.
Dany’s finale appearance hasn’t been shown yet, so it’s unclear what the Khaleesi and Breaker of Chains is up to.
After all, the last time Cersei’s enemies gathered in one place she blew it up with Wildfire.
Perhaps she doesn’t want to reveal that she’s lost one of her dragons, or they want to protect her from Cersei’s tricks.