Let us not forget the "nutty professor". I think these themes work well if they end up focussing on self-acceptance rather than striving for perfection (which often fails). Perhaps if the main character was led to believe that he/she could end up going out with his/her dream person if they lost weight, then as they fail, they find the "right" person while going through their troubles and find peace with their personal reality through that person's guidance...then they fall in love and ride off in the sunset
Anything is funny, what matters is how you make it funny. Fat jokes aren't funny to overweight people, but jokes played on people reacting to the weight would be cathartically hilarious to the undertall in the audience (not to mention that you can make it slapstick which is universally funny). For instance, the snooty girl laughing at the fat kid becomes hilarious as she starts choking on her gum which she's previously been shown smacking and pulling (ick). Then the overweight character who's jogging or something can smirk as the camera follows them away with the snooty girl getting the heimlich in the background of the shot.