Is it? The character is not a stick in the movies, so I don't know what to tell you. In fact Evangeline Lilly added weight and bulked up, looking more curvy, in Ant-Man and the Wasp and in Quantumania. So it character is meant to actually look more curvy than the original Ant-Man movie. And its this version the CMs are playing, not the one from Ant-Man which is probably what you're basing things on.
As we don't know what you saw so its hard judge things, as outward appearance is one of the most widely criticized things on the planet, especially a woman, and everyone's opinion and perception on the matter is different. So we'll just have to take your word for it, but for all we know the CM was perfectly fit and properly cast.