Eating a Devil Fruit could already strengthen one's physique and Haki to some extent, and the Copy-Copy Fruit's ability could also be useful in battle. So what was strange about Charlotte Custard making tremendous progress and completely surpassing her Third Sister Charlotte Amande, whom she had once needed to look up to?
In fact, forget Third Sister Charlotte Amande—even against her stronger eldest sister, Charlotte Compote, Charlotte Custard was now confident she could win. In other words, this young lady, who never seemed to grow up and still resembled an innocent young girl, had unknowingly become the strongest among Charlotte Linlin's thirty-odd daughters!
However, Charlotte Custard had probably reached her limit. Against the stronger Four Sweet Generals, her eldest brother Perospero, the unfathomable younger brother Mousse, or that even stronger fusion monster, Charlotte Custard would not have the slightest chance of winning. The gap was simply far too great!
Simply put, even with Mousse's guidance allowing the Copy-Copy Fruit to provide some help in combat, it still had severe limitations and could not compare with those truly powerful Devil Fruits.
Just as depicted in the One Piece anime, the Copy-Copy Fruit's power could only alter one's appearance; it did not change one's essence. The only way to enhance one's strength and stamina was to grow larger, yet the Copy-Copy Fruit had another limitation: it could only transform its user into creatures of the same species. That was rather unfortunate.
After all kinds of experiments, Charlotte Custard discovered that the limit of her transformations was her mother, Charlotte Linlin. She could not become members of the larger Giant race or the Djinn race, much less nonhuman creatures with even greater species differences, such as Sea Kings.
With Charlotte Custard's height of nearly three meters, transforming into her mother only made her