I think Dormer is not a better actress than Emilia, they both have only one expression. Dormer annoyed me in The Tudors, now in GOT. I didn'r rly like Margaery in books, but in tv show she's not better. Dormer has this kind of face i call piggy face and she doesn't fit to be pretty girly Margaery. But that's just me. I know there are as many fans of her as antifans, just like Clarke.
I don't like when the conversation turns into pitting the actresses against each other, even if it is in this tone.
I think we can disagree or agree in many things, yet the range of some actors, in this show only, speaks for itself, and kinda leaves opinions to the side, and sometimes able to be tainted with personal dislikes towards the person, rather than the actor. Whatever physical attribute the person has, should not be an indication of their acting. Now, if that takes over the performance, then yes, I can “consider” it, though there's nothing to do about it, so it seems pointless.
I think Dormer has a certain range, though her strong suit as an actress, is playing queens/women in power/leaders/influential women. With some variations in her skills to play those, I think she has certain traits of those characters under her sleeve, and can control them well, when they are needed. I’m not saying she’s amazing, I’m not saying she’s Cate Blanchett’s “Elizabeth”, I’m not saying she can play anybody. I’m saying she has enough capability to play Margaery, and that she can certainly take on other roles in the show, because she possess that range. For example: a younger Dormer, could have easily played Sansa. If you had the chance to see Dormer’s work, as a teenager or young actress, you can figure she has played similar roles, and even though I think Turner has a better ability in the broken side of her character, I think Dormer had some other abilities that would have helped her out. (Note: this is just an example).
On the other hand, I think the physical characteristics of the person, the like or dislike towards a character, clouds people’s judgement. I don't like book!Margaery, and I don't care much for show!Margaery, except when she's handling Joffrey, and yet I can recognize Dormer has been doing a good job. I hated Cersei in the books, and yet I can feel the actress is improving in her work. I hate Joffrey, yet Jack Gleeson does an amazing job.
I think some comment like yours was sent by some anon about another actress in the Tudors. From what I’ve seen, none of the actresses who played consorts/wives/lovers in that show, get any good words. Perhaps is the characters, and not their acting. I think Dormer’s acting during the beheading scene, is better than what Clarke has intended to do with her role in this show. And what Dormer has done in this show, is superior to anything Clarke has done. Both actresses had to deal with bad writing, but when it got better, one did good, the other just stayed in the same note.
While Clarke has one permanent face expression, which happens to be no expression at all, and goes from that to overacting, or dead eyes. I haven’t from Dormer’s side, seen dead eyes or overacting. I haven’t seen her stuck in the same expression. I think she’s consistent with one look on her face at times, but perhaps her line delivering and body language gives dimension to her performance. For example: in the scene with Cersei, before Sansa’s wedding, Dormer didn’t have to be that expressive, and she wasn’t, yet the tone she gave to the conversation gave that touch to the performance, leaving the face expressions to Lena Headey (not necessarily I liked them all). I could say, yes, she doesn’t have as much face expressions. Good. But, her line delivering, her body language, the effort in the showing off the traits of her character, give her an advantage to Clarke’s robotic acting.
Even if I were to accept your premise, one of them has given second hand embarrassment and cringe-worth performances throughout 3 seasons. While the other started in this show badly, or in an average way, and yet came to own her character and the scenes, during this season. And that’s undeniable. I'm not a total fan of Natalie's acting, like I said, she started in this show with a barely passable performance, but in this season she grew, and was able to make justice to the better material provided. I'm not in love with her acting. I think she has improved, but I wouldn't consider her the best anyway. Clarke doesn't have range, goes from dead eyes, overacting, to a robotic performance. She's been around 3 season now, and there's no real improvement other than controlling a bit more the line delivering when she speaks valyrian. Like I said in the beginning, while the proof one has range and the other hasn't, it's on the show. From line delivering, to a wider range of face expressions, from knowing her strong points as she flaunts the character's traits, to body language, and overall presence in the scene.