It was/is also that way in Athens, at least in the "anarchist" quarter of town. Every Friday the so-called anarchists (would) take dumpsters and light them on fire, to make a point, that the government is corrupt, or something. I'm not Greek so I can't speak for Greeks but other Greeks told me it's just part of life, so just look the other way. Also trash collection seems to often be an issue. They also have an island just for trash, but I heard it's already overflowing so they are looking for another island to use as a trash dump. Admittedly I haven't been there in a couple years, so maybe things have changed lately, but I do have a lot of photos of dumpsters on fire.
This is not really accurate. I have lived in Athens. There were some years where burned dumpsters were not a rare sight (after the big riots of 2008), but it is really, really uncommon, especially considering the size of Athens.
And I have never heard anything (or were able to find something on google) about an island full of trash. Islands are simply too valuable for that, and you'd also need to bring the trash there by boat.
When did you last live there? I was traveling/living in various parts of Greece for short periods of time over the last 30 years, the last time in 2018, where, due to EU economic sanctions, there were still regular fires happening in the anarchist district (Exarcheia). Googling shows that there are still regularly things burning there (as well as Zografou), though lately[0] it seems to be primarily vehicles. Just Google "Exarcheia fires" and you will find a lot of articles from almost every year going back to at least 2008.
Zakynthos is the island I was referring to. It's an illegal dumping ground, and even the EU has fined them for it. Also Kalymnos has a decades-old issue with burning trash - they even call it "trash volcano". Lots of other islands also have illegal dumping issues. Also many nature areas in mainland Greece are not in the most pristine state, often littered with junk, often junk that you can tell has been there for a long time. Whenever I went exploring by car, we would find the most beautiful areas but almost always trash everywhere :( Maybe it's gotten better though, I haven't been back since 2019, though I'm not sure I really want to.
[0] https://www.thenationalherald.com/arsonists-run-amok-in-athe...