Let us take a look at how APA does this. Their rule, from their official style blog, is to cite anyone whose contribution is "essential to understanding the content".
For example, the illustrations Sir John Tenniel made for Alice in Wonderland are not essential to the text. The book has been published without them and can be read without them. Therefore Tenniel is not mentioned when you city Alice.
In a comic book or graphic novel on the other hand, the illustrations are an essential part of the content. Without the contribution of the artist, the comic simply didn't exist. Therefore, what the APA team recommend, is to
"cite a [comic book] as you would cite a non-illustrated book with more than one author".
As I find their logic convincing, I would recommend the same approach for a citation in MLA format. The MLA Handbook is not explicit about this case, but since it doesn't explicitly forbid to list the artists as second authors, that is what I think best represents the importance of the artists in a comic book.
In European comics, usually one artist does it all. In Japanese manga, one mangaka represents his whole team. But in American comics, the roles are clearly divided, and I would list the penciller, inker, colorist, and letterer as authors after the writer.
After all, an author is "one that originates or creates something".