^^ If it's your own contest you can do whatever you want. Grammar doesn't have to be something you care about since you're the one in charge of making a rubric.

If you're making your own contest, there's no need to put in grammar as a criterion if you don't want to. As long as you make it clear by what you mean about "enjoyment", like, do the characters need to be well-developed? is there a logical flow to the story? is it understandable even with errors?, that should be fine