In terms of narrative impact, Sylvanas is tied to worse, less coherent stories and along with Anduin has resulted in the current wasteland that is both her own race and the entire Horde. But if judged purely as a character, it's a different story.
Sylvanas has gone in several shit directions but she has at least gone somewhere. Jaina has only gotten more and more powerful while ending up exactly where she started on poor reasoning and her story has stayed solely in Square 1. Every fault that can be levied at Sylvanas, from schizophrenia to ridiculous power level to the narrative twisting to allow her to succeed and fuck over her enemies can be aimed more so at Jaina, with the added benefit that Jaina's story has next to no tie in what what passes for the overarching plot. Sylvanas took a backseat in her own Garrosh-ing expansion. Jaina is not only the only raid boss to win unreservedly, but is backed by her opposing faction in the very next appearance and then pops up in Azshara instead of the Night Elves and in all of Shadowlands instead of Genn, Tyrande or literally anyone who has the least bit of a meaningful tie in to death or opposing Sylvanas.