The anime wanted to have a girl character that is tolerable to look at instead of Misty (or any of Ash's companions) being acne-ridden and "ugly". If she was ugly, not many people would want to even look at Misty, and in turn, would stop watching the Anime.
Pretty girls also attract the bulk of an anime's audience (males) and this is displayed in other animes as well.
Also, kids take loving to a pretty or grandmotherly character, or real life person. This is why many early-grade teachers are either pretty, motherly, or grandmotherly.