I think it's fair to pin the failure of Holdo's plan on Poe, not Finn. Finn and Rose brought the information and some ideas to Poe, but it was ultimately Poe's decision to undermine the leadership. Finn was not a mutineer, he was the guy sent to carry out the plan. The failure is not on him, he did the best with the situation he was in. He still has an impact on the outcome, he and Rose are charged with carrying out the plan.
However, I don't think it's entirely fair to require him to have had an immediate impact on the event anyway. His arc isn't about changing the world, yet, it's about become the kind of person who can change the world. He was still drifting, having found someone to fight for, but not yet fully engaged with the cause. He has the clearest arc in my opinion, ultimately demonstrating that he is willing to sacrifice himself to be the hero. That he doesn't succeed, and is ultimately stopped by Rose, doesn't change the fact that his character has evolved.
It's okay not to like the events surrounding Rose and Finn's characters in film, to consider them clunky or poorly written. However, I don't think it's fair to say they deliberately threw Finn under the bus or did him dirty. Rian Johnson didn't have a mandate to write a bad part of the movie, and then decided Finn would be the sacrifice to the alter of mediocrity. That whole section seems like what Rian would believe is the adventurous beating heart of the film, and he gave it to two severely underrepresented minorities of different genders. It's clear it didn't work for everybody, but I think it would be disingenuous to suggest it wasn't a try.
However, I can understand the idea of wanting it to be better. Finn is still a great character and Boyega is the most energizing actor in the films. I wanted more Finn too. I think the film ultimately left him in a very good place to actualize that desire.
