"Low Fat" often means "High Sugar" which is often junk from a nutritional point of view. Neither of those are what you want.
If you're just pointing out that people have subjective opinions of this sort of thing. Well then great, but that doesn't add too much to the conversation. One person's junk is another person's gizzard salad.
There are many quacks and quack diets out there, so I don't know quite how to establish an objective standard for diets that are tailor-made for people to avoid junk food. We have rough measures of the amount of nutrients we need to eat per day. So maybe we can point at a rough consensus from world-wide experts?
I propose that foods that overwhelm those nutrient levels in the wrong way; Say adding too much fat, sugar, sodium/salt, etc, be labeled as "Junk Food" and taxed lightly so as to adjust the perceived price difference between fast-food and healthy food.
It's a bad cost to society to have to support people in self-destructive patterns, it's a literal monetary cost, and we effectively incentivize the behavior that gets them free healthcare. A counterweight has to be applied to keep people at the same effective equilibrium point in health. Societal communal healthcare has it's problems, but if we don't want to just be throwing money down the drain, we have to use strong motivators to help people regain or maintain their health.
If a person can demonstrate that they won't be a burden on the shared societal health plan, then it should be a right to opt out of the plan. But opting out should be a waiver-worthy process. If you opt out, and then at a later date get sick, you can't just opt back in. -- Avoid the free-loader tragedy of the commons.