Yes a loss of appetite is a common symptom of the flu.
When you have the flu your body may feel hungry but you may not want to eat at all or very little.
Your body is using more energy to fight off the flu and your body is likely telling you that it doesn't want any food at the moment so you lose your appetite.
When I have the flu I get hungry which is normal when we don't eat but I have to force food down my throat because it seems like my body is saying no don't feed me.
But I try to eat some Ice Cream, Chicken Noodle Soup or any other easy to consume foods.
I still need to eat to have at least some energy but there are some times when I have the flu that I cannot eat for at least a few days.
But I still drink some water, orange juice tea etc to stay hydrated.
Once the flu is over you should regain your appetite.