I've noticed a decline over the last few years as well, mainly in the interior smell. I'm an Avis First member and rent from only a few airports

-Last week PHL - 14k miles Ford Fusion, smelled like an ashtray and didn't have "no smoking" stickers in it (thought Avis was a smoke free fleet?)
-EWR - have found pop bottles and other trash rolling around in the back seat on full size and premium SUVs
-all airport Avis locations - have had many cars that smelled like ... Chinese food maybe? How do so many cars get the same odor?

I generally don't complain about cleanliness or smell, but it is getting to the point where I might start to. I also used to get a lot of newer cars (under 5k miles), but lately I've gotten a lot of older models with 15-25k miles on them that feel very "broken in"