Maybe a little of both? I mean if she wasn't doing a good job it'd have been easy to drop her and bring in a new character the following season, so she's clearly doing something right. Given the choice it's doubtless preferable to keep an actor they like working with if at all possible.
On the other hand, a bunch of other characters have also have their plots and character traits merged and/or switched around compared to the books. Ashford probably being the other big change as 'Book Ashford' was a vain, obsequious coward and a braggart to boot, who isn't heard from again after 'Abaddon's Gate'. 'Show Ashford' seems to have been completely overhauled with traits from the book version of Bull (who was the XO of the Behemoth, not Drummer) but also quite a bit that feels entirely new.
I generally take this kind of thing as an extension of what I think I mentioned upthread: that because of the authors' close involvement, the show feels less like an adaptation and more like a series of revised drafts of the books. The story is still basically the same but he execution is generally much better.