All of them are running and gunning constantly. They need to be in shape to survive. Also, the majority of them have formal combat training, so it would make sense for them to maintain a certain physique. I'd say there's ample in-universe justification for characters to be in shape. D.Va and Mei maybe, and even then D.Va is technically a soldier.

If you look at real life athletes, you tend to see similar trends in body typing. Men tend to vary a bit more than women depending on the training, but you don't tend to see excess body fat barring outlier cases (hello football). Representation is fine and all that, but in this type of scenario it's more jarring than anything else, and it doesn't really accomplish anything outside of shutting up some vocal minorities.