The nymphs are possibly inspired from Dante manuscripts

[Koen G]

Pierre, just a year or two ago, I would have written the same as you. But I've had to change my mind. At least part of the reasons why the images look the way they do is that the artist(s) was not very skilled. 

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. By this we mean: untrained, unpracticed, unskilled. Someone who is not good at drawing, for whatever reason. Someone who could not produce a much better drawing even if they spent some more time on it. 
2) The drawing was done in a hurry or with disinterest, regardless of the artist's skill level. Doodle, quick sketch, rushed job...
3) The viewer is not used to the artistic norms and customs of the artist's culture or time period. There's something "ugly" about early medieval art, for example, but blaming this entirely on unskilled artists would miss the point. As children of Renaissance values, one of the things we prize in art is a certain realism, so when we see art with different priorities, it may be jarring.
4) The artist knowingly goes against current norms and expectations. A lot of contemporary art is not easy on the eyes and distorts forms, but that's the point.

It has always seemed logical to me that the VM art falls in category 2 and 3. There's a bit of a rushed feeling, especially when a lot of nymphs have to be drawn, which gets worse as the Zodiac series progresses. And people who call it ugly may be judging it by the wrong standards.

With a drawing of a plant we don't realize, we can't judge the drawing quality. But with animals and people, this can be done more confidently. The biggest red flag is that poses and perspectives become more tortured as they deviate from the cookie-cutter forms. And 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. 

This is just the first area I happened to zoom in on to find an example. Ask: 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.

   

We can eliminate (3): clearly the artist does not have a cultural or stylistic preference to draw arms like rubber tubes, because we see just as many sharp elbows. It's all inconsistent. And I'd say we can also eliminate (2) here: the individualizing details on the barrels and clothes suggest that this wasn't a rushed or disinterested job. 

Importantly, drawing a plausible basic anatomical frame is a very low bar. You will notice that this is usually done well, with the right amounts of bends connected to roughly the right place. Therefore, I would say the artist wasn't very skilled, and only trained to a certain degree. Enough to work with writing and drawing utensils, but not (yet) enough to pose arms and legs.