Because she's chubby doesn't mean at all that she doesn't care or don't give a fuck about her work or whatever. What is that logic even. It's not that easy to lose weight for everyone - and even if it would be, some people tend to gain more the more stressed they are, no matter how much working out they do or what they eat. And she's just a kid. Puberty, anyone? Ugh. Were people this bitchy about Kago and Tsuji gaining and losing weight back in the day? Cause if they were, I could easily see why Kago would get depressed.
Kanon is by no means hugely overweight and even if she is a bit on the chubby side, the one to blame is the goddamn stylists that don't do fuck to make her look slimmer. But instead put her in things to "cover up" her figure or, like in One, Two, Three they don't give a damn that the style of the outfit she got made her look about three times as big as she is.