In several (at least 4) of Shakespeare plays, someone is addressed as 'my preserver' or something along those lines. In those cases, the person who was so addressed had rendered a service such as saving the other person's life or saving their kingdom or something of equal magnitude. When I read that line, it makes me think that Margaret, who, as we know, has "had already imbibed a good deal of Marianne's romance, without having much of her sense" is imitating something she has probably heard, as Marianne would, but on this occasion it's too much.

Willoughby brought her home with a sprained ankle, but it's not really the equivalent of having saved someone's life. So, it's with more elegance than precision to say this of Willoughby.

I did find online a book that is a collection of letters and other writings by William Cowper (one of Jane Austen's favourites whom she made a favourite of Marianne's too). In one 1783 letter to his son, he writes "Simplicity is become a very rare quality in a writer...where refinement in all the arts is carried to an excess, I suppose it is always rare." In the same paragraph of the same letter, he also criticizes some writers for having a "want of elegance" in their writing, but he thinks others have too much elegance. Some, he says, are too straightforward in what they write and others make minute observations about things they could not realistically know anything about, so they are lacking in precision.

Overall, he seems to be arguing for a balance and he criticizes writers as 'censurable' for using too much 'false ornament' in the way they write.