Appeal has more to do with the character designs in animation. It is the basic idea of designing characters that are distinct from each other and attract your attention.  Appeal is easier to understand if you have an understanding of silhouettes and shape language.

Silhouettes are in short, the understanding of what the shadow of the characters your drawing would look like, if you design characters for your animations and they have similar silhouettes they are not very attractive to the eye. Silhouettes are also a way for the audience to identify the characters, to increase appeal it would help if the characters are distinguishable by their silhouettes.

Shape language is similar, but more in the concept that each character is built from many different shapes. If the characters you design have a lot of similar shapes you could lose the appeal of the character. Shapes have different understandings to them; you can tell a lot by the shapes and lines. When thinking of soft and kind characters you likely would associate them with a rounder appearance, harsher shapes can also be true for harder or evil characters.

Shape language and silhouettes are the basic understanding of appeal, one can mix the two subjects. In addition to the two, you can also add effects and different colors in order to create a better visual to the characters or environment. You could also play with contrast and create characters that have a harsh appearance, but a kind personality.

https://www.dsource.in/course/principles-animation/appeal#:~:text=IDC%2C%20IIT%20Bombay-,Appeal,They%20should%20have%20charisma.

https://pixune.com/blog/the-principle-of-appeal-in-animation