Unhealthy diets affect only the people who follow them, not their peers. Smoking is different because passive smoking does affect others - that's why there have been government interventions (higher taxation, public campaigns) to reduce smoking in many countries in the last decades.
Your argument is besides the point anyway: just because there are other factors in life that can make you sick, that does not mean that we should not fight COVID.
Your argument is besides the point anyway: just because there are other factors in life that can make you sick, that does not mean that we should not fight COVID.