It's subjective of course, but Mara does seem to have been more popular overall. I mean Lucasfilm never bothered to hire an official Thrawn "model" and one assumes they only did so for her (and no other character) to meet the demand for Mara Jade merchandise. She was also around in a lot more stories. Thrawn's still pretty up there in terms of popularity (even before his canonisation.) Indeed if one were to do a poll of people's favourite EU characters, I think it's a safe bet both Thrawn & Mara would be somewhere in the top 5, if not top two or three alongside Jaina Solo.
I'm not a big EU guy by any stretch. But, Mara is a highly popular character in and out of canon. She got a couple of action figures, and a lot more ancillary material appearances. I generally don't care for much of the EU, but calling it "garbage" is dumb. It had its ups and downs, just like any other property. Also, somehow Jaxx made it back in...that's odd.
Sorry, but I hated the Thrawn trilogy and think most of the EU is pure trash. That said, I thought Thrawn was a decent character and I was pleased to see him used well on "Rebels." And I'm not arguing that Thrawn and Mara weren't "popular." Of course they are/were. Doesn't mean I want to see Mara show up in a "canonical" SW production like, ever. Yes, but have you see the hair?????? http://www.rebelscum.com/Hasbro-Star-Wars/The-Black-Series/Figures/14-Mara-Jade/photo-archive.asp
I never read the books when I was kid, so my first encounter with Mara was in the Jedi Knight expansion pack 'Mysteries of the Sith' in 1998, and shortly after that the 'By the Emperor's Hand' comic series. Which I do still have, though some of the pages have seen better days, and one issue is missing the cover/back page. I have since read the books where she was introduced however.
There are lots of things I don't want to see in a canonical work but that doesn't mean I don't automatically reject them out of hand. It's all in execution. Her appearance in "Mysteries of the Sith" is probably my favorite.
They showed almost half an hour of footage to the press and so far most of the reactions on Twitter have been very positive.
It's not mentioned in that video above, but apparently Carl Weathers confirmed (off-camera I guess) that he's going to direct an episode in season 2. (Technically, I don't think season 2 has even been officially officialized yet.) https://twitter.com/Breznican/status/1185639082627493888 WARNING: the video a few posts above does get slightly spoilerish in the second half, so if you want to remain spoiler-free on some easter eggs, just wait until November 12.