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.
In a fanfic where Alvin is in prison in the 'Alvin and the Chipmunks' universe, it could be a really interesting take on his character. He might face challenges from other inmates who don't like his upbeat and sometimes mischievous nature. However, Alvin could also bring some positive change to the prison, like starting a music group among the inmates to pass the time and boost morale.
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.
One possible reason for Alvin's depression in a fanfic could be the pressure of constantly being in the spotlight. As the lead chipmunk, he always has to be on top of his game, whether it's performing or coming up with new ideas. This unending stress might lead him to feel down. For example, in a big concert coming up, he might feel like he's not good enough and start to doubt himself, which could be a start of his depressive state.
In Alvin x Simon fanfiction, they might go on an adventure together. For example, they could get lost in the forest while trying to find a special type of nut for a recipe. During their time lost, they rely on each other's skills. Alvin uses his quick thinking and physical abilities to get them out of some tricky situations, like crossing a small stream. Simon, on the other hand, uses his knowledge of the forest's flora and fauna to find food and shelter. Through this adventure, their bond grows stronger.
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.
In an 'Alvin and the Chipmunks' Alvin prison fanfic, Alvin could face a lot of difficulties at first. He is used to a life full of freedom and fun with his brothers. In prison, he might be lonely and scared. But as time passes, he could learn to adapt. He could teach the other inmates about music and harmony. His positive attitude could gradually change the prison culture. He might even find a way to communicate with his brothers outside, through secret messages or smuggled notes, and they could be working on a plan to get him out.
A great Alvin smart fanfic might be where Alvin figures out a way to save the Chipmunks' tree house from being demolished. He uses his intelligence to find a legal loophole or comes up with a brilliant plan to raise enough money to buy the land it's on. For example, he might organize a huge concert with the Chipmunks and use the proceeds for this purpose.
One possible plot for such a fanfic could be that Alvin gets separated from the group during a concert tour. He might find himself in a small town and has to rely on his wits to get by. He could meet new friends who initially don't believe he's a famous chipmunk but eventually help him reunite with his brothers.