The smallfolk were already in for a hungry winter; now, many of them may starve, and all the dragon fire in the world won’t be able to feed them.
Even if Dany’s goal isn’t to make the common people suffer, they’ll likely suffer all the same, perhaps even more than they would have if she’d just stayed on Dragonstone.
The people of King’s Landing may begin to starve a lot sooner than they would have if not for Dany and Drogon, to say nothing of the people of the Reach whose crops were taken by the Lannister army.
Even if that does happen, however, the fact remains that Dany and Drogon just burned a whole bunch of supply wagons, and we’ve heard time and time again in Season 7 that supplies are already running short throughout the Seven Kingdoms.