Is there a massive drop in quality of Cretaceous vs. Triassic dinosaur fossils?

This proably cannot be answered in a scientifically valid way at this time. There are hundreds of thousands of fossils across that span of time and no one has looked at all of them, "quality" is not something that is measured routinely. Worse there is a massive confounding factor, different formations have drastically different preservational quality. For instance the Solnhafen is a much earlier formation than the Lance formation but has drastically better preservation amd detail just becasue of the drastically different environments.

Even trying to study this question would be swamped with problems just due to how drastically different the quantity of fossils are from each time period. Quality for fossils is strongly influenced by the local formation conditions (chemistry, grain size, burial speed, ect.) so it may be nearly impossible to answer without doing a specific study( it would be very expensive study). I am aware of no such study ever having been done and I know of no generally recognized difference but this is just one paleontologists experience.

No, there is no massive difference in fossil quality due to the extra 200 million years or so that the oldest dinosaur fossils have spent in the ground compared to the most recent ones at the end of the Cretaceous. Once fossils are properly fossilised, they remain unchanged unless something happens to the rocks. This could be weathering, which damages or destroys many valuable fossils, including recent ones like hominids, or it could be metamorphism, which damages or destroys them with heat.

I have seen a photo of a one-billion-year-old fossil from the late Precambrian of a marine algae reliably dated by the new Rhenium 187 - Osmium 187 method, and it was in an excellent state of preservation. Likewise with fossils preserved in amber, some of which are tens of millions of years old but look as fresh as if they had only died yesterday.