It honestly depends on how it's handled. There's nothing wrong with eating salad, using hair dye, or wearing contacts. So if it's handled as a "be true to yourself" story, I think that's fine. Maybe she was trying to be someone she's not, so she gives it up and becomes the chubby nerd.
Many of these popular-to-nerd stories tend to go another gross route, unfortunately. Sometimes, the popular girl has a fall from grace (usually a revenge plot) and becomes "ugly" (aka a fat brunette nerd). Other times, she's an air-headed bimbo that's "improved" by becoming a fat brunette nerd.
Both of these versions are misogynistic. The first one is extremely fatphobic.