The centre of the borough of Croydon, around East Croydon and West Croydon stations, is horrible, for many reasons, but, as others on this thread have said, this is only a small part of Croydon itself, which is one of the biggest London boroughs.
A fact about Croydon which seems to have been overlooked on this thread is that it has the site of the first passenger airport in the world, so that Croydon became prosperous in the 1920s and 30s, and lots of businesses moved there. This is also why there is a lot of 1930s housing in Croydon. I live in a 1930s house, and so does my daughter and her family. The airport also started running commercial flights, and was seen by the Germans as a significant target, so Croydon was bombed relentlessly during the second world war, and lots of it was destroyed.
After the war, particularly during the 50s and 60s, there doesn't appear to have been any strategy about rebuilding Croydon, or planning controls, and that was, I believe, what led to the foulness which is central Croydon now. There are lovely little pubs and houses which survived the bombing overshadowed by horrible skyscrapers, flyovers and multistory carparks!