Lettuce, tomato and onion are not unhealthy. The cheese and meat may possibly be, but the real culprit is the bread and possibly any "special sauce" that's added.
A burger with lettuce tomato, cheese and onion isn't alone unhealthy. It's unhealthy to eat too much of it or to eat too many fries with it. It's also unhealthy to have a beverage that has too much sugar with the meal.
My doctor told me while I was pregnant that he considered pizza a good meal as long as it was a supreme with all the veggies on it. Because it has all the food groups. (I did/do not eat that kind of pizza....but the advise still shocked me)