Just a simple visualization using boxplots of IMDB ratings from the past few decades, of the 80s, 90s, 00s, and the modern day from 2010 and beyond.
Some values of the data actually had few ratings so I cut these out (anything less than 20k) since I reasoned they did not have a big enough sample size.
It is important to note before drawing any conclusions that IMDB ratings are in no way an absolute measure of quality, as not all moviegoers frequent IMDB. In fact, one can reasonably assume that impassioned movie-goers with strong opinions, positive or negative, are more inclined to rate movies online than the average person.
The 80s do not have a lot of stinkers according to the visualization (unlike the 2000s), but that also could be due simply to a lack of data (or lack of voters). Admittedly there are a lot more movies that have been reviewed from the 2000s and beyond.
On the surface it does appear however that public opinion of films has gradually decreased. Opinion of modern films seems very high conversely but there is a substantial lack of data considering a good part of the decade is still yet to happen.