Ledgers portrayal would be considered average if he hadn't died. Just my opinion.
How much you follow canon doesn't have much to do with it in my opinion. It's not difficult to dislike JJ's interpretation of the Star Wars universe, because he made it first and foremost part of his JJverse... TFA didn't feel like a star wars film, it felt like an Abrams film.
Once I got past Kylo taking off his mask in TFA, and that initial hit that he's just a whiny little emo bitch, I've actually come to appreciate Drivers performance as one of the best things about the sequel trilogy.
Ledger's passing has no effect on how I view his performance. I love his little movements and tics, his Tom Waits-like vocal affectations, his laugh, his line delivery, his grungy make-up and attire...he embodied a principled anarchist (an oxymoron if there ever was one) sowing chaos and fear in those who were near and far.
In the third portion of TDK, though he does falter, but I think that's moreso of the entire movie faltering from that point than Ledger's performance. In my view, whenever he was on, he commanded my attention and makes me wonder what the third movie in the trilogy would have been like as Nolan had originally was intending had he not died.
We tend to look back on performances whether they deserve the accolades they have received, whether they be actual awards and/or plaudits, and we often have that sense of buyer's remorse. I have none regarding his portrayal. I think he earned every bit of praise that was heaped on his performance.
Now whether Phoenix's performance is near or on par, that's yet to be determined. But considering his own sense of dedication to the "craft" (jeez, that just sounds so bloody pretentious), if there's anybody who could, he could. The trailer makes it look promising.
Now regarding the other kettle of fish that is Star Wars, I speak of those who tend to focus so much on the lore that they get bogged down by it, and I don't say that as a bad thing. Since I've never really been that much of Star Wars fanatic (I enjoy them, but I've never really venture outside of the cinematic releases), I am not so burdened by it. You can't be annoyed with something if you have no basis to be annoyed by it, right? There is a part of me that would like to delve deeper into the whole lore, but with less free time available to me, I think that possibility has passed. With TFA, it just felt like narrative re-treads from the original trilogy. At least with TLJ, there were newer elements that builds on the over-arching themes. I just felt less bored with TLJ.
And regarding Kylo Ren, it's not so much an issue with Driver's performance. It's more that the character of Kylo Ren just lacks presence. I just don'get that sense of feeling of being threatened. There's nothing larger-than-life about him. Maybe that's the point...those who want to do great harm don't always wear masks or are ghastly disfigured beings. Evil an be in plain sight.
Man, I went "full nerd" with thos post.