How can I prevent malnutrition as a normal person who has little time to cook?

Picky eater? More common problem is over-nutrition --> obesity, not malnutrition. Unless you have a medical disorder, malnutrition is very hard to accomplish. With modern conveniences such as grocery stores and microwave oven, you should be able to prepare a balanced meal from scratch in 10-15 minutes, about the same amount of time it takes for drive-through fast food. Gourmet meals will of course take longer.