The US federal deficit is, order of magnitude, $1.5 trillion dollars annually. A billionaire has assets, order of magnitude, of a few billion. Even if they just confiscate their entire wealth of all the billionaires then it will do approximately nothing compared to the vast bulk of the US government. I think people underestimate how large and centralised the US State has become since WWII.
If this is being pitched as strictly a class warfare measure then sure. It sounds like a bad idea but hey, politics away. However if the idea is to actually tackle any of the problems that the US is facing then this scheme seems pointless on the face of it.