While I don't hold the idea that the Hugo award winning novels have gotten worse over the years the late 80's does give someone enough material to build a case. While there was only one book that I disliked in the period there is a distinct feeling of treading water.
That comes from the fact that this period is dominated by sequels to other Hugo winners. The love for Ender's Game swept in Speaker for the Dead while The Uplift War and Cyteen continued their author's universe. Even Neuromancer shares a character in common with Gibson's earlier "Johnny Mnemonic". Even Ender's Game was a fix-up of an earlier novella of the same name. There was not a single original work in the lot.
An odd thing is that all of the winning authors are definitely products of the 80's. Besides her two novel wins Cherryh had one award for a short story in 1979. Orson Scott Card won for his two novels as well as a novella in 1989 and finally got one more award for non-fiction in 1991. All three of Brin's Hugos were in the 1980's too. Even if Neuromancer was not clearly a product of the 80's it is Gibson's only Hugo award. This is a perfect example of how the awards reflect the shifting tastes of fandom; they were the "hot authors" of their day and the situation changed rapidly.
If there's one theme that dominates this period its pulpy space opera. Strip away the re-examination of the interface between humans and machines and Neuromancer is a noir detective story. The Ender books are almost straight pulp despite the psychological themes. Brin makes no pretense of his Uplift books being anything other than an updating of space opera. Only Cyteen breaks away from this mold.
The tally board for the novels currently reads:
Didn't Like: 12
The only book I didn't like this time was Speaker for the Dead leaving me with a solid two to one majority for the books I've liked.