Fanfiction authors are people who write as a hobby. They’re not paid, they do it on their own time, and they do it for fun. Some authors use fanfiction as a way to improve their writing, but unless they ask for critiquing comments, don’t be that person - even if you have good intentions. You don’t see the damage that you do, but damage is done.
The best way to encourage fanfiction authors to keep doing what they’re doing is to let them know what you liked about their work. I’ve seen too many fic authors get discouraged in their writing because of people who leave less than favorable comments on their work. Leave the critical comments for people who get paid to write.
Again, I’m not asking you to lie to spare the authors feelings, I’m asking you to just refrain from leaving a negative comment.