I wouldn't worry about it too much. My daughter is four, and has gone through these phases many times. In my opinion, as long as she's eating fruit, yogurt, and other healthy things, she's not doing any harm to herself. If she refuses to eat anything but potato chips, twinkies, and stuff like that, then I'd say you have a problem. My daughter LOVES fruit. She would eat strawberries for every meal, every day, and love it. Many times she's gone through phases where she doesn't want the meal that I've prepared, she just wants fruit, or yogurt, or cheese. She tends to snack throughout the day, instead of eating big meals. In the end, they say its actually healthier to eat several small meals throughout the day as opposed to one or two big meals, so again, I never worry much about her dietary habits, as long as what she's eating is healthy, and her belly is full. Children also go through growing phases where they seem to eat anything that isn't nailed to the floor, and then other times they eat like a bird, pecking here and there at various things, but never really eating a full meal. Even though I know its frustrating, even at this age, your child knows when she's hungry and when she's not. She may just be going through one of those post-growing phases where she's simply not that hungry. Also, it is said that when you crave a certain food, its because your body lacks and needs a certain nutrient in that food. So this may be the case, also. Her body may need more of the stuff thats in yogurt, cheese, and fruit, and less of the stuff thats in mashed potatoes, chicken and rice, if you catch my drift. With my daughter I make "deals"...such as....if you eat five bites of chicken, I'll give you strawberries for dessert. Or if you eat three bites of each item on your plate, you can have some yogurt.
Also, if you're really adament that you want her to eat her meals, instead of just snacks, stop giving her snacks between meals. Or stop giving her snacks altogether, so that she's hungry at meal time, and therefore will eat.
What you choose to do and how you handle it is up to you. The bottom line is this: Eating fruits, yogurt and cheese isn't going to hurt her.
Good luck!! :)