I find that that really depends on the manga artist. Some artists I've seen with consistent quality, but others like Akisha Ikeida of Rosario Vampire have actually improved over time.
Then others like CLAMP are wildly inconsistent. take a look at head to body proportion just from one page to another. not even different chapters.
Not that I find any of this bad, I just think it's a little harsh to say that only animators make all the goofs.

