Puzo is in no way a bad writer (no famous author is, but everything is relative--ie. he could be "bad" compared to someone else) but he isn't a classic author. Well, he could be classic in that he wrote the novel on which the best film ever was based!
The book was badly written IMO. But the fact is that it is a good novel in terms of plot and such, but in terms of literature, TGF was appalling IMHO!
mick