I don't mind if a character is nice or not, doesn't affect how fond I am of them at all. I love Ramsay, Joffrey and Roose despite them being monstrous human beings seemingly without empathy. The characters in Abrams' Star Trek are all heroes, yet I wish they'd all die in a horrific fire
I don't mind a character being "unlikeable" at all. Not sure when it became a problem really. Macbeth and Othello are hardly likeable in the traditional sense.
Tyrion's ADWD chapters are weak because they're padded fat that dominates the book. Sometimes GRRM tunnels way too far in to a narrative dead-end. I love this series in large part because of the "filler" that isn't essential to the main plot, but I'd prefer more interesting filler. Tyrion's chapters weren't awful, they're well written enough. There's just too little story to justify that much time being devoted to a minor plotline.
The first 3 books have such strong structural elements. The last 2 feel weirdly random in pacing and structure, which is in stark contrast to something like Lord of the Rings which I think is perfectly structured. People on Westeros always bring up the behind the scenes story of the Meereenese knot, or the abandoned 5 year gap but.. they're terrible non-excuses. Just because a book has to be the transition book doesn't mean it has to be padded out with lesser material. The middle part of a trilogy is frequently the most praised. The second act shouldn't be when the pacing dies.
To use an example everybody knows, The Empire Strikes Back
is the transition film with no real beginning or end. Yet, it doesn't just spend an hour meticulously showing Luke travel to Dagobah in order to fill runtime. The storytellers actually came up with interesting stuff to happen, despite the characters being put in place for the next movie.