Well, it could be many things. Maybe the writing is just really bad, like full of grammar and spelling mistakes. Or perhaps the author completely misinterprets the characters from the Zelda series, making them act out of character. Another reason could be that the plot is really convoluted and doesn't make any sense in the context of the Zelda universe.
The Grinch is a fictional character. He is a green, furry creature who lives on a mountain above the town of Whoville. He hates Christmas and decides to steal all the Christmas - related things from the Whos in Whoville.
One reason could be really poor writing skills. For example, if the grammar and spelling are constantly wrong, it makes the reading experience very difficult. Also, when the plot doesn't make any sense at all, like characters suddenly changing their personalities without any good reason, or the story going in completely illogical directions. Another aspect could be a lack of originality. If it's just a copy of other popular fanfics with no unique elements added, it can be considered awful.
He probably saw or heard about similar real - life situations in the neighborhoods he grew up in or was involved with. These experiences inspired him to create the story of Brenda.
Since we don't have the full details of this 'you got a friend in horse troy story', it could be that the main idea is centered around the adventures and experiences that are shared between the person and the horse Troy. Their relationship might be the key element that drives the plot forward, with them relying on each other and learning from each other throughout the story.
One reason is that they don't match the child's age group. A story that's too advanced in terms of themes or language for a young child is going to be awful. For example, a story about complex political intrigue isn't suitable for a preschooler. Also, if the story is too preachy, trying too hard to teach a lesson in a really obvious and dull way, it can be bad. And if the illustrations, if there are any, are really ugly or disturbing, that can also make the bedtime story awful.
The Grinch is purely fictional. He's become a beloved character in children's literature and media, but he doesn't exist in reality. His antics and personality are all part of the fictional world.