I’m sure there were buildings with nice artwork on them. Ironically, communist countries had a well defined ruling class who could get nice things. It’s the average that counts. Also, slapping a small artwork on an otherwise ugly building is not really a solution.

My experience is mostly Romania, Hungary and ex-Yugoslavia. Those in Romania and Hungary are awful undecorated gray boxes, with substandard installations, paper-thin walls, and very depressing places to be in.
Yugoslavia is a different story, visibly not a part of the Soviet bloc. While there are some bad buildings, on average both quality and aesthetics, are much much better. Fairly nice places to live in