Is imagining cartoon images in one's mind, with modest clothing but without a head covering, in stories free of forbidden things, considered haram or an imitation of Allah's creation?

There is no sin in what you have mentioned. A Muslim should be careful to guard their heart from being overwhelmed by vain imaginations. The beginning of every theoretical knowledge and chosen action lies in thoughts and ideas, which necessitate perceptions, then intentions, which in turn demand action. It is not possible to eliminate thoughts, but the strength of faith and intellect helps in accepting the best of them and repelling the worst. Correcting thoughts is easier than correcting ideas, and correcting ideas is easier than correcting intentions. The most beneficial remedy is to occupy the self with thoughts about what concerns it. Beware of allowing Satan to take hold of the house of your thoughts and intentions. The heart is never devoid of thought, whether it is about the Afterlife, or this world, or whispers [from Satan].