We unfortunately don't know much about Ghen's writing ability during that time. Did it truly progress as you suggest? We don't know. Riven, when first written, seemed stable as well. It WAS stable for a long time.
Riven was written about 10 years before he began teaching Atrus. I don't remember if it was ever mentioned what number he was working on at that time, but Ghen had at least 10 years of experience (and who knows how many Ages) writing at that time.
Plus, the ridicule wasn't entirely because of his haphazard writing technique, but also had to do with his beliefs and mental health issues. He thought he was a god, actually creating the worlds he wrote.
I would also point out that he was the only one of this time to have actually received ANY D'ni training. Granted, it wasn't much, but it was more than either Atrus or Catherine received.
As for you assertion that Ghen didn't have good materials. You're right to the point of the time he's trapped on Riven. However, that didn't happen till 1774, about 20 years after he first returned to D'ni. So he had access to the same good materials for that entire time that Catherine and Atrus had later.
Ghen never struck me as the kind of person who could really learn from his mistakes. That goes for his relationships as well as his writing. Ghen has LOTS of issues, his writing technique the least of them. For me, the ridicule is justified.