NutriScore is mostly useless, to the point of being misleading. The system was cooked up by the industry, which explains a lot.
It is a label that tells you how nutritious a given product is "compared to products in the same category". So you could have, say, candy or frozen pizza with a NutriScore A and that would be just fine according to this system because it happens to be more nutritious than other candy/pizza. In other words, a product having a NutriScore of A doesn't mean the product is actually healthy or good for you.
We have a traffic light system, pretty useful. But when all items in a category are bad for you, and you know it, them all having red lights doesn't help much.
I'd certainly try alternatives that are marginally healthier, if that's true generally then it puts some pressure on food industry to move to healthier choices.
European NutriScore "assigns products a rating letter from A (best) to E (worst), with associated colors from green to red. High content of fruits and vegetables, fibers, protein and healthy oils (rapeseed, walnut and olive oils) per 100 g of food product promote a preferable score, while high content of energy, sugar, saturated fatty acids, and sodium per 100 g promote a detrimental score." https://en.wikipedia.org/wiki/Nutri-Score
Nutritionists.
Labels on processed food products go in the style of "Contains an excess of sodium", "Contains an excess of sugar", and so on.