Since you did not tell me what you mean by "mental disorder" or "conventional dieting," I will assume that "conventional dieting" means attempting to lose weight and keep it off by means of eating a balanced diet which is restricted in energy content.

While engaging in (and maintaining) such a diet requires a lot of thought about one's food intake, I don't see any basis to categorize such thought as a "mental disorder." It need not make one miserable; undermine one's relationships with family or friends; or stop one from holding down a job.

So basically I think you need to re-think this idea particularly if you yourself are trying to lose weight.