This is the definition of desease: "a particular abnormal condition, a disorder of a structure or function, that affects part or all of an organism". It is very clear that obesity does indeed meet the standarts. However, I don't think fat-shaming is the solution. Would you insult a person with cancer just because they have cancer? Would you bully a person with anorexia to "help motivate them to gain weight"? If not, then your insulting obese people is extremely hypocritical.
Obesity is a major problem in today's society. The fact that obese people are being more and more accepted to society doesn't mean that there isn't something wrong with it. Obesity causes other diseases but it doesn't make it less of a disease (e.G. MS causes paralysis(a disease) but is still a disease).
Obesity is a mental disorder unless it is caused by diabetes. A few extra kg is healthy because it keeps heat in the body and can also be used when starving, but when you see someone with more than 100 kg (I'm not from the US and obese people have more than by double weight) then you know those lives are destined to a lot of other diseases, higher blood pressure, less mobility, more injuries etc.
Suggesting that obesity is a disease is then saying that obesity is something that one can be born with, can receive, or can happen randomly like cancer. That is no where near the case, while yes there are some disorders like, hypothyroidism, however they are extremely rare! A disease is not caused from the eating of too many fatty fast food. Our society is becoming more and more use to larger people, we are even thinking as labeling as a disease, we should be making it more uncomfortable to become larger. The reason for this is because of the endless health problems that being obese brings.
The main problem with making obesity a disease is not any of this, it is that now a company can be sued for firing a larger person, even though the cost to make a larger desk, a larger workstation may out weight the cost.
Being obese is an unfortunate lifestyle, if you choose to eat fast food and not work out then you must live up to the backlash of that. Society should have that stand point and not make it easier to become fat.