Natalie Portman in the first Thor movie was great for me. The scene where she phones Selvig and says "That thing you told me not to do...I just did it" was extremely memorable, because there's personality and risk to the way she delivers the lines. Now I'm honestly very ignorant of good and bad acting, so I don't notice any difference in TDW for her performance, but I really like that scene from Thor. It's got character, and that's a rare thing for modern movies.
My issues with Jane in TDW are numerous: her romance subplot was annoying, the supporting characters she brought with her added nothing and only took away from better things, she only came across as histrionic and unlikable when she hit her supposed boyfriend (it's not a big deal to me to normalize DV in that way for laughs, but it turned her character into an emotional monster), her role in the final battle was pivotal and that would have been fine...except it actually overshadowed Thor's role...which might have still been fine except Thor had spent the entire film getting relegated to a background character and only looking lame in his own movie. Taika was right that a movie with Thor in the title succeeds or fails by how well they are presented as the main character.
There's a lot of stuff centered around Jane in TDW that brings the movie down, but I don't see most of it as being the actor's fault. There is a fair bit around the director Patty Jenkins getting fired over creative differences and the gender politics behind the scenes, which definitely hurt the making of the movie. Natalie is definitely accountable for some of that, since she raised a huge stink about Patty being let go. Ultimately I think it's for the better that Jane (and Natalie) were gone from Ragnarok, but I think the original movie showed how Jane could have worked, with just a few edits to her character. Show her taking over from Selvig, and the student surpassing the teacher, etc, and go from there.