it helps to give examples haha since my brain immediately went to Thumper, but I know what you mean.   I would say it has to do with the way male-ness and cis-male bodies are often interpreted to be the “default” and female characters need to be made “feminine” in order to be recognizable as females.   This happens in language also- most people will refer to an animal of unknown gender as “he” unless the animal has a smaller animal with it, in which case it becomes “she” and “the mommy.”   Also recognizable is the way this extends to how we’re taught to indicate gender with symbols- it’s pretty common for children to be taught to indicate female characters with a hair bow while boys are usually given no markers (they are the default).