Well, wet dirt is permeable and my understanding was they needed to stop the radioactive material leeching into the water table. Which would have caused radiation poisoning and cancer rates to soar for much of Europe.

It also was not stupid from start to finish. If it were, it would still be uncovered and half of Europe would need to be evacuated.

IMO the biggest problems were to do with the reactor design being secret. That and the attempted coverup afterwards. But I wouldn’t consider the cleanup to have been incompetent. The Chernobyl liquidators have been widely credited with limiting the effects of the disaster.