I've found plane cleanliness to be pretty hit or miss and not really reliably good or bad with any one airline I've flown. I think it's to some extent luck. A 'normal' passenger will at worst leave an empty package or wrapper on their seats so that is pretty easily picked up so if you get that seat you'll probably not think of it as dirty in any way even if the cleaning staff wasn't very thorough. If there's a huge, obvious mess, it will probably also be cleaned and quite aggressively so, simply because it's so obvious even lazy cleaners won't be able to get away with ignoring it. The tricky bits and the ones sure to ruin your perception of a plane's cleanliness are gross things that don't immediately stand out and might be in places not easily reached. The 'filth' in the seat back pocket is probably a classic there and indeed the grossest thing I ever found on a plane was a used tissue in that pocket. That was on KLM for what it's worth.
I think there's a correlation between age of the fleet and perceptions of cleanliness. I think it's because new feels clean so if we aren't unlucky enough to find something obviously gross, we'll think of the plane as clean no matter what. There's also the factor that it takes some time before filth in hard to reach places reaches critical mass, a new plane likely isn't there yet. Perhaps staff including FAs also feel a bit more motivated to keep a new plane clean. It's sort of like the 'broken windows' effect of cleaning. If something looks immaculate you don't want to be the one to ruin it whereas if something already looks grimy you probably feel like no-one cares either way.