Given what the Soviet Union has done to its people in the past, it's believable. Of course, it's also believable that Ukraine did it. And it's also believable that some agency of the United States did it.
No, it’s not believable that Ukraine or the US want to destroy Ukraine. It’s perfectly within historical realm that either Russia neglected the dam to the point it collapsed or that it simply was blown up by Russia in an orgy of Stalinist collective punishment against Ukrainians. There is plenty of historical precedence in Russian history for both options there.