The nymphs are possibly inspired from Dante manuscripts

[Jorge_Stolfi]

There are four reasons I can think of why a drawing may look "bad" to a modern, Western viewer: 1) The artist is not good. ...

I agree with most of your analysis. But I would not dismiss item 1.

Starting from the Aries pages I see that the Scribe's nymphs were terribly crude at first, but he gradually learned to draw more decent ones over the next half a dozen pages.   And You are not allowed to view links. Register or Login to view. looks like it was drawn even before Aries.

And I'd say we can also eliminate (2) "The drawing was done in a hurry or with disinterest, regardless of the artist's skill level. Doodle, quick sketch, rushed job..." the individualizing details on the barrels and clothes suggest that this wasn't a rushed or disinterested job

I agree.  I would not call any page a "doodle, sketch, rushed job", even if many details look that way.  My guess is that each page took him about one hour for the text and about one hour for the illustrations.

But I do see signs that the Scribe became somewhat sloppier as he progressed along the Zodiac, even while he learned to draw better nymphs. And he seems to have been more careful on You are not allowed to view links. Register or Login to view. than on all the other Herbal pages.  

Thus I would say that, after spending extra care on the first pages of each section, he relaxed a bit, but maintained a consistent level of care from then on.

As for the barrels and clothes, my theory is that he drew them in order to hide the awful job he did on the first few nymphs.  Or just to avoid having to draw those parts.

3) The viewer is not used to the artistic norms and customs of the artist's culture or time period.

To me it is clear that the Scribe did not know the artistic "language" of the time.  On the "Spiral Galaxy" page, for instance, he seems to have misunderstood  the cloud-band of the original drawing as the edge of a pool or plate.

Thus I currently imagine the Scribe as a young uneducated guy who had some writing experience -- but on paper, not on vellum, and for purposes where legibility was much more important than beauty.    Before he took on this job, he probably had not seen anything like those books that the copied  -- Oresme, Taccola, Balneis, etc.  

And he probably was not being paid much.

He may even have been the Author's teenage nephew, who had a nice handwriting and agreed to do the job for only a couple of bucks per page...

Even on pages where poses should be consistent and the artist clearly spent more effort, there are big differences in how successfully human anatomy and proportions are represented.  ... where are the elbows? How long is each part of the arm? Does the arm come from the shoulder or the belly? The answers are different every time.

But keep in mind that those nymphs are very small, about 20 mm tall.  It  is very hard to draw an anatomically correct human body at that scale, without first sketching it in pencil (a trick that Scribe obviously did not know).  Trained artists like Leonardo (or even Taccola) could do it.  A small-town accountant or lawyer probably could not.

All the best, --stolfi