Dandelion greens are fairly nutritionally dense. They are high in vitamins and minerals, good for bones, blood and liver.
The vast majority of people who might have access to—or want to eat—dandelion are not malnourished and do not have vitamin deficiencies, and so adding dandelion to their diet will have zero net effect on their health. Swapping some vegetable out for dandelion will have zero net effect on their health. Replacing one doughnut with one portion of dandelion will have zero net effect on their health when compared to swapping out that doughnut for any other reasonable vegetable.
Individual foods are, generally speaking, neither healthy nor unhealthy. Diets are healthy or unhealthy.
It is funny the ideas that become popular, isn't it.
Consider also fresh food deserts that exist in many cities. https://en.wikipedia.org/wiki/Food_desert
Dandelions are widely available in most cities and very easy to grow, guerrilla gardening style. https://en.wikipedia.org/wiki/Guerrilla_gardening