I think that the data is showing the game to be fairly balanced in approach with two exceptions:
Ackbar, Rieekan, Motti, and Screed are the most common overall admirals and most common at the top by a good amount. It would be nice if the dispersal of admiral choice and effectiveness was more even.
When it comes to ships, most are showing up in 15-25% of winning fleets, which is a much more even dispersion. Demolisher Gladiators are the sole exception, as they show up in nearly half of the winning fleets. It definitely sticks out.
I think Demolisher, Motti, Screed, and Akbar are simply ease of use. They have a very straightforward effect that doesn't require much additional consideration. Rieekan's dominance for the Rebels I think has everything to do with the meta. He is hands down the best answer to the Demo rush which drove this forward into a tizzy.
For the life of me I can't understand why Mon Mothma doesn't do better. Our #5 at Martinsburg was Mon Mothma and only a single tourney point behind me before the cut. I'm actually very glad I never played him because I don't think I could have done that well against him. I designed and play my list to ram straight down the throat of enemy fleets. That works well against Imperials and Akbar, but not fleets that disperse their combat power. I see Mon Mothma (and soon to be Cracken) swarms as my natural counter.
I agree on the Admirals. They are the easiest to use and get their benefits from. I also am puzzled by why Mothma has done so poorly. I've used her to great effect and she work wonders in many cases. She isn't showing up much and when she does seems to occupy the bottom 25%. I am also surprised at the lack of Vader. I know there are varied opinions, but I really like him and find him easy to use. It is really surprising how scarce he has been.
My sense is that as far as peformance goes, Mothma at least is statistical noise. If only 5% of the overall fleets (121 in the chart currently), then we're talking 6 total fleets. We don't know how many of those are really excellent players and how many of them are outstanding. Mothma is perhaps the admiral I'd most like to get going, but I can say that when I saw Clon's Demolisher list reach prominence, that my builds really started from the standpoint of "What is my answer to a high activation, high bid demolisher?" This meant I gravitated to Rieekan. The other key answers are to outactivate it, which is at least possible with Mothma, or to outbid it (something I'm just not interested in doing). On the other hand, squadrons and Rhymer have a big part in the meta, and that's an area where I think Mothma responds well. I think to some extent it just takes the right good player toying with Mothma long enough to get some good magic out of it.