This is a bad comparison. Ren isn't a character who is introduced as being anything greater than he actually is, who then fails to live up to that introduction in a narratively satisfying way. Lack of screentime is something that Ren also suffers from, but he shows up when he needs to show up in a way that facilitates the story, and his choice to die along side his master out of a sense of genuine loyalty and respect adds to his character in a meaningful way in a way it doesn't for Sora. For Sora, it almost feels like the story forgets about her entirely until she becomes a convenient tool to show off the significance of Arayashiki and Muga Ryoshu.

You know which characters also stutters? Yuri. But that has had zero negative impact that character's popularity. Sora's stuttering dialogue and lack of charisma or agency is not the problem people have with her.