I guess it depends on which way you look at it.
Like one of your other repliers stated, it can be a 'result' of other things. It can be a result of bad habits, not being knowledgeable on what is healthy food, or being a food addict. For a rare few, it can even be genetic or a result of other diseases such as hypothyroidism, diabetes, or other hormonal issues.
Then again, obesity can be looked at as a disease if you look at the definition of 'disease.'
A disease is defined as:
"A disorder of structure or function in a human, animal, or plant, especially one that produces specific signs or symptoms or that affects a specific location and is not simply a direct result of physical injury."
"A particular quality, habit, or disposition regarded as adversely affecting a person or group of people."
Obesity definitely does fit that definition. Obesity, in most cases, are not a result of physical injury but from one's own disorder of structure. Their particular habit includes a variety of things: bad nutritional intake, overeating, and being consumed with a wrong mindset.
Thus, most diseases wreak havoc on the body, and obesity definitely does this. It can create other health issues like diabetes, heart disease, and inflammatory and autoimmune diseases such as arthritis and fibromyalgia.