It's no more a disease than being a smoker is a disease. It can lead to a lot of diseases though.
It's a bit rich to blame individuals entirely though. Yeah obviously if they really put in a lot of effort, they could probably lose weight and get fit, but we've created a world where it takes more effort to be healthy than unhealthy. Especially in North America. My sister lives in Canada and you basically can't get anywhere useful without a car. Every shop is in a massive retail park so you can park right outside. Everyone goes to work and spends most of the day sat on their arse. And then they go home and eat processed food where even the low fat options are laced with sugar. I mean sugar in kids' milk. When did that happen? And obviously the solution is to eat fresh food, but people opting for convenience is mainly a result of no longer having one member of the family staying home to prepare fresh meals, so it does take more effort and planning to shop for and prepare a fresh meal every day after an 8 hour shift. So yeah, there's a bit of personal responsibility involved of course, but it's also massively down to urban planning and the food industry that place personal convenience above everything else. It's not a secret why countries where it's easy to cycle, everyone cycles to work and they don't get as fat. Whereas if you want to cycle to work in the UK, there are more immediate risks that heart disease.
As for a mental illness, well I'm sure there are a minority of people who have underlying issues, but the explosion in obesity, and on a wider scale people generally being a bit fat is not caused by mental illness. If that was true, then we'd be saying that mental health issues have massively increased over the past few decades, and coincidentally, it just happens to have occurred in the countries that eat the most processed shit and get the least exercise. But don't worry everyone, we'll have a free trade deal with America soon, so we can look forward to it getting much worse. Look at what happened to the Pacific island populations when they started importing huge amounts of processed food. Up to 50% of adults obese. 30-40% diabetic.