Not mutually exclusive. My pet conspiracy theory is that Soviet buildings were somewhat intentionally made to be awful, not so much to save on resources, but because the appearance of “suffering for the revolution” was desirable. Literally - we can’t let people have nice things, it uncommunist.
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