A still useful definition of eating healthy is to eat approximately the correct amount of calories.
Obviously the what is also going to have impact, but it is easy to observe that the great majority of people are not walking around with severe nutritional deficiencies.
Debatable. They clearly have enough calories, but you need more than just that to live and be healthy. I would strongly argue that omega-3's and vitamin D3 are essential nutrients that a substantial portion of the population are deficient in.
I said severe because there are clear complications for severe deficiencies (rickets, scurvy, etc.). The science surrounding correct levels and consequences of not quite correct levels of such nutrients is much less settled.
So among the group of people that are not basically falling apart from a nutrient deficit, the ones that eat about the right amount of calories tend to be (quite a lot!) healthier than the ones that eat far more or far less calories.
Obviously the what is also going to have impact, but it is easy to observe that the great majority of people are not walking around with severe nutritional deficiencies.