It would be wonderful if the world worked that way - unfortunately it doesn't.But like, can't there be a fat character without the whole plot revolving around the fact that they are fat? It feels like cheap pandering. Why can't they just make the usual movie about friendship and magic but the main character happens to be fat?
Especially for a child/teenager, being chubby/fat is a formative part of their life (... it is really different for people who gain weight as adults). No matter how privileged the circumstances - whether in material or immaterial terms - are in which a fat child grows up, it will somewhere down the line always get the message that there is something "wrong" with him/her, that he/she needs to change to be "normal". And that there are almost no role models portrayed anywhere that can be followed.
That is a type of adversity a young person needs to overcome - and it has long-term impact on personality and psyche.
As long as this remains the case in society, the plot-lines will not change. And as many mentioned here, having a positive fat character still is so rare, the depiction itself qualifies as a success to be celebrated.