Overweight person tax? I say NO for systematic (not moral) three reasons.
1) To execute the tax you'd have to get the BMI every adult person living in the country. What about immigrants? Tourists? Sounds hairy...
2) Tax exemptions for medical problems would be insane. Think government offices are busy now? Just wait.
3) What if someone loses a bunch of weight? Gonna give them a refund? Or are you going to take their weight from before to determine the tax? In fact, on what day will the BMI be decided?
Unhealthy Food Tax
Pro 1: This would reduce the consumption of unhealthy food.
Con 1: Unhealthy food, for those on the bottom, is better than no food.
Pro 2: Food producers are easier to tax than individuals.
Con 2: Determining what food is "unhealthy" could be tricky.
Pro 3: Taxes on unhealthy food would help shift producers to making healthy food.
Con 3: Likely, most producers would go as unhealthy as possible until they hit the point right before a food would be classified as "unhealthy".
Overall, I say BAD IDEA.
Souce: Econ major and did a research project on the topic.