Yeah, the artwork is hit and miss a lot of the time, though I haven't seen all of the chapter artwork (doesn't each chapter have its' own little picture to go with it?) Her mother's art isn't Chris-level, obviously, but it's not great either. For example, let's look at the woman's face:
Sorry about the line down the middle; it didn't match up perfectly and I think I got the bit of the monster face in my selection too. (I actually didn't realize it, but the cover picture seems to be on an angle?) As you can see, aside from the fact that this woman seems to be still living in the 1980s, what with her huge hair and excess makeup, the proportions in general just seem
off -- sure, her face is divided up into thirds as it should be, but stuff like the size of the eyes, eyebrows, and lips just seems too big or too small for a face of that size. Maybe, and I'm being generous here, the artist only drew the half of the face and didn't intend for it to be mirrored, but that just makes it look even worse.
And I'm seconding what MysticMisty said about the hair -- in this case, less is more. I see a lot of artists make that exact same mistake, where they think that realistic hair means detailing
everything. Not to mention the way it falls is just weird to me -- it's relatively close to the head at the top, but seems to get bushier as it falls down the side of her face. I'm not even going to get into how flat the face looks -- she didn't need a lot of shading, but some shading around the bridge of the nose, where the eye sockets would be, would do wonders in making the nose appear less flat.
I'll admit, the monster side is kind of cool though -- the snake hairs (I assume) remind me of Medusa. You could probably say the scales on the skin are too detailed, though to me it doesn't come off as glaringly wrong because it's not real, if that makes sense.