Obesity is a negative for both the individual and society. The fat acceptance movement is terrible because being fat isn't some sort of permanent, genetic condition that's out of people's control. You control what you put in your mouth and how much exercise you get, it's a choice.
Should we have an ignorance acceptance movement, where we don't push people to educate themselves? That's a choice as well.
The fact remains that 95% of people going on diets to lose weight aren't successful in keeping it off long-term and the only method that seems to show significant success is surgery. I find it hard to believe that the only problem here is that fat people aren't aware they're fat. Harping on it in the way you're advocating just discourages people from doing anything healthy, since, you know, if getting slim is all that matters, why bother exercising if you're still fat at the end?
Should we have an ignorance acceptance movement, where we don't push people to educate themselves? That's a choice as well.